So this question comes up a lot in the QnA forums and its got to the point where I copy and paste my answer, so I figured a small guide would probably suffice. I'm sure there are plenty like it but hopefully I can add a few things and give some insight to do things differently, this is just a quick brainstorm I came up with in 5minutes for the sake of the answer. 

1. Buy items in the city
Easiest and quickest to do, there are plenty of items in the city shops, you can buy 100 a day, the buy price on many of them is cheaper than the sell price. Go check them out for yourself. (You can buy 100 a day from the city). Sell to other players, item market or Bazaar

2. Trading
Similar to the first suggestion, but buy from other players, sell to other players, through trade, bazaar or item market. Look for items that are always in demand, xanax, FHC, meds etc (buy low sell high). This can have the biggest potential. There are players that earn Billions doing this. It requires a lot of time and effort and it is a skill you must develop

3. Blood
Do the education course in the biology sub-section that unlocks to ability to fill blood bags, not only can you sell these but you can also use them yourself to save money on Morphine

4. Sell Losses - Slutting
Selling losses to other players is a great way to make some money when you first begin. I wouldn't rely on this as your staple income though as it will soon start affecting your battle stats. Losses tend to go for 200-400k each. Attacks cost 25 energy. You can make four attacks without donator status, so each full energy bar you can gain an easy mil. If you add xanax into the mix you can do 10 more attacks. That's another couple of mil earned minus the cost of the xanax ~800k. 

5. City map
Check the city map now and again, zoom right in and have a good scroll around. (treat it like a lottery; you're unlikely to win big, but its nice to be part of it). You can find a wide variety of items simply sitting on your map, I've personally found a skateboard (440m). My spouse found a welding helm (1b) and a couple in my faction have found items such as Cesium. Its worth looking now and again but don't hold out for it.

6. Basic Jobs (Revive / Spy)
Reaching the top rank in the army or medical careers open up a couple of job specials that you can utilize for monatary gain. Reaching the top in the Army will allow you to trade job points to spy on players battle-stats, other players will pay for this ability, though you may need to advertise in the trade chat or appropriate forums. Reaching the top of medical unlocks the ability to revive people, the pay is normally 1 xanax or 1m per revive. Revives as standard cost 75 energy, but if you get yourself in a reviving faction this is reduced to 25 energy. 25e for 1m is good money.

7. Travelling
Simply put, once you hit level 15 you unlock to ability to fly. You can fly to far away lands to buy goods, then you bring them back here, to Torn to sell to other players. There are plenty of guides on travelling so I'm not going to go into much detail. You should look into renting a PI at this point, Buying a Large Suitcase and potentially getting a Bazaar. The profit you earn from travelling will cover the rent of the PI. There are also jobs and factions that can increase the amount of items you can bring back = more profit. 

8. Graphics
So you're pretty good at photoshop? Why not put your skills to use, there are plenty of players out there that make good money making other players graphics for their profiles, their factions and their companies. This is personally how I made my first few hundred mil. Big disclaimer on this one, the world of the graphics forum can be an unforgiving place, don't go in there and try and sell a stock image with some text overaly or a filter applied. Actually put some work in, learn how to make decent artwork & ask for critiques before you attempt to sell anything.

9. Banking
The city bank offers a place to keep your money safe with a returned profit. You cannot withdraw until the banking term is over. Typically players won't suggest this route unless you can maximise the the profit you're making from it by putting 55 merits into banking interest (10/10) and maxing out the amount you can deposit (2b). This is a great passive money maker when you do manage to get to this point.

10. Donator Packs
You can purchase donator packs with real currency and sell them in game. Not too much to say about this, its roughly £3 : 25m

11. Giveaways
There is normally always some form of giveaway going in the forums or there are some discord channels you can join that do a lot of lottos / giveaways.

12. Casino
Feeling favoured by lady luck? Got a particular talent for Blackjack, Poker or skilled in the bookie? Then give the casino a whirl. I'd warn you though, house always wins, if you take a look at the all time statistics, house is ahead in every single game. Be prepared to lose more money than you make. I honestly just debated not putting this one in this guide, but there are players out there with pretty good success stories with the casino, but they are rare.

13. Bounties
Bounties are pretty competitive and I've never really made much money from them personally, but I'm sure if you have the battle-stats to back you up and you're vigilante and active in the listings there is potential to make some decent money here.

14. Stock market
The stocks are a great place. You can either play them as a trading game, buy low-sell high, or you can simply invest your money into the different companies and hold what is called a benefit block. Benefit blocks are where you hold enough stocks in a company to earn a benefit. SYM will pay out drug-pack every 14 days, FHG will pay FHCs every ~14 days. I'd highly suggest looking into these if you have plenty of spare capitol laying around. They can be great passive money makers. 

15. Mugging
Mugging randoms for the sake of mugging will most likely not be that lucrative but believe it or not there is a whole science to it, from buy-muggings, mugging abroad or stalking casino players. Again there are plenty of guides out there that talk about these more specific strategies so I won't go into any detail.

16. Player owned Jobs
There are plenty of player owned companies out there that will pay good money for some decent working stats. Not only that but if you look for companies such as Adult Novelty there is a lot of potential to earn money from the job specials. 

17. Scamming
Allowed but i'd highly advise against it. There are multiple methods that I've heard of, but again no details from me on this one. Be warned if you do go down this route you'll most likely end up seeing the inside of the hospital way more than the outside.

18. Attacking 
There is a whole world of mercenary work out there. After all one of the biggest mechanics on this is the ability to attack other players. People often pay decent sums of money to have other people attacked, to aid in territory wars, to assist in missions and many more. check out the attacking forums.

Torn has a lot of different oppurtunities when it comes to making money and if you  put your mind to it I'm sure you'll find mulitple ways you can start your grind to becoming a multi-billionare. The above was simply my 5 minute brainstorm on different money making techniques. With a little creativity, Entrepreneurship and determination anyone can make plenty of money. The two biggest traits to a succesful player is consistancy and patience. 

Good luck in your endevours

Riot