This is basic Torn advice that everyone needs to know. Many things in Torn are not intuitive, so I'm trying to get you moving in the right direction.
Reaching level 15 is very important. Level 15 allows you to travel, and when you can travel, it becomes much, much easier to make money. Most people reach level 15, and then start running flowers and plushies. You fly to another country, buy flowers or plushies, and fly home. Then you sell those flowers or plushies to another player. There is a very big market, lots of people want to buy them, and you'll get a very nice profit for it. You can easily make $3M a day or more doing this, more if you are very active. Some make $5M-$6M profit a day doing this.
To reach level 15, you need to train your gym stats a bit. Hopefully you've been doing some of that, but you may need to do more. (And long term, even the people who have played for years still use most of their energy to train their stats.)
Once your stats are high enough, there are some targets known as "leveling targets" that you can attack. These are people (inactive now) who have fairly low stats for their level. They are easy to beat in a fight. You attack them, then leave them in the street. That gives you the best experience gains and lets you level up fairly quickly. I have a list of leveling targets in the next msg of this thread.
Once you can easily win against the leveling targets, you should use all or almost all of your e (energy) attacking them and leaving them in the street, until you reach level 15. If you can't beat them easily (ie, if you lose or if you win but take a lot of damage) then you should train more then try again in a few days. Once you reach level 15, level isn't important any more, and you should go back to gym training with your e. Level always helps, but you don't need to actively work on leveling up past level 15, just let it come naturally, and put your energy into training in the gym.
Happiness plays a big part in gym gains, especially when you are small. That's why the shack isn't a good place to live. Low happy means low gym gains. I usually recommend renting a mansion or a ranch until you reach level 15. Don't buy a property, and don't rent long term. 30 days is about right.
Once you reach level 15, you can easily afford to rent a PI (Private Island) and there are several good reasons to do so. Almost everyone rents a PI when they reach level 15, and some who can afford it start renting one sooner. That's why you don't want to buy a cheaper property and you don't want to rent a mansion or whatever for long term if you can help it. PI's with 3600 happy are usually the sweet spot, where you get the most for your money, and they usually rent for roughly $12M for 30 days.
But as a brand new player, you probably can't afford to rent a PI, so often a mansion is the best choice you can afford. At a minimum, rent a penthouse with 925 happy. Don't bother with anything with lower happy than that.
When you first move into your mansion, your happy won't change. It will gradually go up to the max happy the mansion gives you, but it takes a long time. You can buy lollipops at Sally's Sweet Shop, in City, East Side.) Using lollipops will raise your happy. Use them to reach the max happy for your mansion, then stop. If you've moved into a Private Island, then using lollipops won't get you to the max happy right away. You could consider using an ecstasy, which costs around $70,000. Using one will double your happy.
Your "max" happy isn't really a max. You can go over, even way over, for a short period of time. If you are over your normal max, then Happy resets to that max every 15 minutes. xx:00, xx:15, xx:30, xx:45. Any hour, every 15 minutes, it resets.
You can use lollipops to raise happy above normal, and then use an ecstasy to double your happy. That gets your happy much higher than normal. Then you use the gym train your e and get much bigger gains than normal. On the next 15 minute mark, happy will reset, but you can train the e away first.
Once you can afford it, you can do a much more extreme version of that, called a Happy Jump. Those are expensive and take two days to complete, but they give very large gains. I'll explain those in another post further down.
It's hard to make big money until you are level 15, but you can make smaller amounts of money by going to City, East Side, and shopping at Bits N Bobs or Sweet Shop. Lollipops resell at a high markup. In Bits N Bobs, beer, or any of the basic plushies, will also resell at decent profit. You can buy 100 items a day in the Torn City shops, and reselling those items should make you around 50k to 60k a day in profit. Trading can be profitable, and can be done before level 15, but it takes money to make money, so unless you already have a fair amount of money, it's hard to make much profit. And when you are small and new, you are easy to mug, which makes profitable trading harder. Trading can also be very time consuming, so it may not work for everyone. There is a sub-section of my guide specifically about making money.
You will get mugged. Everyone gets mugged. Learning to protect your money from muggers is important. There isn't a perfect solution to that. Vaults are great, but very, very expensive. Ghost trades work well - but they have a six hour limit. Some people store money in faction vaults, but there is some risk, and you usually don't have immediate access to get your money back. Some buy items or points, and store there money in those, reselling them when they need cash. Most people will probably need to use a combination of several methods to store money, at least until they can afford to buy a Private Island with a vault, and quite likely after that, too. I have a post in my guide about storing money.
When you first reach level 15 you're almost ready to start making good money, and you can stop actively trying to level up. Beyond level 15, levels help some, giving you some merit points at certain spots and extra life for each level. But from here on, you should level up naturally and not spend your energy actively trying to level up. Spend your energy training in the gym. Once you can travel, combat stats are much more important than level.
Rent a Private Island as soon as you are level 15 (if you haven't done it before.) You need a PI with a landing strip and a pilot to travel efficiently. A PI speeds travel by 30%, and allows you to carry an extra +10 items. Those extra items mean more profit on each trip, so the extra profit you can make is more than the cost to rent a PI. The higher happy will also help with your gym training. There are PI's with higher happy, but if you're just getting going, a 3600 happy PI is usually the best deal, the best cost/benefit ratio. Any PI will work, as long as it has a landing strip. To rent, visit City, Estate Agent and then click on Rental Market. Once you can afford it, add a large suitcase to carry an additional +4 items, but the PI should come first as it will have a larger impact.
Put your merit points into education time reduction first, until that is maxed. Then into banking interest rates until it's maxed.
For education classes, finish any class you are already doing, but then start working on Sports Science courses, and take all of those. Those are highly recommended as the first place to get a bachelors degree.
Some people recommend taking the Introduction to Biochemistry and Intravenous Therapy classes first, because the two of those will give you access to blood bags, which have several advantages. I prefer Sports Science classes first, but I think it's a close call.
If you simply want to read lots of advice on various subjects, the game forums tutorials section has tons of information. This post is a list of various guides/tutorials sorted by category, which may help.
Missions: When you first start, Torn will encourage you do do missions. And the first missions you are given are designed specifically for new players, to help you learn the game. Basic things like buy and equip a weapon, attack a specific (very easy to beat) target, explore the city, and begin doing crimes. It's useful, and helps you get a feel for the game and learn the basics. However, you may want to wait on those missions. Many of them give additional energy when you complete them, and if you hold off on the missions, you can rent a property where you will have better housing, which gives higher happy. And if you train in the gym with higher happy, you get bigger gains. So you may want to wait before accepting the missions, to get a small additional gain from the energy that's given as rewards for these missions. It's entirely your choice, and not a big difference. You can find details on the new player missions here. Once you've completed the new player missions, you will be able to move on to the more "normal" missions. Long term, those let you buy weapon mods and special books, which you can't get any other way. But missions can often use a lot of energy, and shouldn't be considered high priority, especially for newer players. Training is more important. Personally, I tend to do most of the missions I'm given, but there are certain ones that I just skip because I know they tend to waste a lot of energy.
The next few posts in this thread have more advice, on specific subjects.