Like the others said, I’d like to reinforce some points.
*don’t start a biz with your friends as partners, or employees. It’s very risky. What happens when your friends don’t work as hard as you need them to? Do you sack them? Sack your friends? Big ask! Most people will let the company limp along, and make excuses, when really, it’s clear they need to let their friends go, and employ new people. It’s better if you never put yourself in this position if you can possibly help it. Accept help from friends, maybe even pay them for specific tasks… but don’t employ them.
- Working for someone else to learn the ropes for at least 12 months is a great idea. Of course, you learn industry specific things, but you’ll also learn basic but often un-taught things, like dealing with debtors and creditors, accounting, “check is in the mail” mentality, employment issues, and a million other tiny things that you’d never think of on your own. Watch every single thing your employer does, then make your own decision on if it was a good idea or not (but don’t tell him! :)). It’s not essential that you work in the same industry as your chosen area, as the principles are the same everywhere, but obviously it’s helpful.
What other people said:
Don’t do anything that doesn’t have the capability to produce an unusually large amount of profit.
In theory, this is a good idea, but in practise, not many people make a killing on everything they do - or even most things. There will be high paying jobs, and low paying ones, and ones you make a loss on. Of course, you want to get as many of the high paying ones as possible, but the reality is, as soon as someone perceives you are making too much money, they’ll start (or change) their own biz to do the same thing for cheaper.
The other tact - depending on what you biz is exactly - is to produce something that multiplies out easily, and sell lots of them, making a small profit on lots of items, rather than large amounts of profit on a few. That’s a pretty basic concept, but not thought about a lot.
For example, if you go to a printing company to get some biz cards printed up, you might notice it’s $50 for 250 cards, and $58 for 500 cards. $8 for 250 more cards?! This is because, they incur their expenses in setting up the press to print your cards in the first place (artwork, bormides, films, etc), not the actual printing of cards themselves. The actual paper and ink to print your cards on is dirt cheap. In fact, the printing company I know only ever prints in batches of 1000, and if I only want 500 cards, they just throw the other 500 cards out. Sounds crazy, but it’s more efficient in their case you put card in that suits 1000 cards.
The best part of having my own business is that I can set my own hours - I get to work whichever 12 hours per day I want!
Sure, in an ideal world, but usually, your hours are dictated by the hours your suppliers and customers keep. Unless you do something like computer programming or pottery, I think you’ll find yourself doing something like eight hours a day, 10 to 6, or something similar. Of course, you can take breaks, and start late, finish early, and so on, and maybe that’s what NBIT33 meant.
With regards to money… I think having enough saved up to live off for 12 months is pure fantasy (regardless of how useful it might be). I guess it depends on your circumstances and lifestyle. I started my biz while I had a full time job in another industry, and it was basically a hobby that grew into a part time job, and from there into a full time job. For a while I was doing two full time jobs, and both suffered, but it meant I was able to quit my old job, and run my own biz from home, and not have a dip in income. That was cool, and in hindsight, worth it (tho damn hard work).
I’d recommend starting to work on your biz well before you quite your learning job. Setting up things like bank accounts, accounts, law issues, biz registration, and so on can take two months for someone not skilled, and while that’s going on, you’ll not be making much money, so you’d want to have some kinda income.
I was lucky when a friend recommended an accountant for me, who was more of a business advisor, in a way. He helped me set up my biz in the correct legal way, and made sure all sorts of obligation were met (workers insurance, taxes, fees, dozens of other things that I had no idea about), leaving me to do what I knew best - photographing naked people. 
Good luck if you can find someone like that!
Lastly, there are lotsa people out there who can help (including ppl here on the SDMB). Like others have said, listen to as many ops as you can, and then make your decision.
Good luck.
