As with many things, a lot of it depends. Everyone has posted some good things to think about.
Another thing to consider is whether you really mean to be profitable or if you just like working on stuff and sort of want to break even. If it is going to be profitable, you're going to have to run it as a business - whether above board or under the table. Anything else and you likely won't turn any profit.
This decision affects your question about charging friends. If it is run as a business, you have to charge friends. You might give them a discount, but if you are too generous where do you draw the line? This friend only pays for materials but that friend pays half your hourly rate and someone else pays your full hourly rate? What happens if they compare notes? Then what happens if you have a paying job and a friend wants something done as well? Do you put off the friend since he isn't paying? Do you turn down the paying job? What happens when you have your own project, overtime, or other family obligations? A paying customer isn't likely to understand if you have to drop his project for a couple weeks. Unless you are considering it as a business, a lot of what you'll be doing is going to be word of mouth with friends anyway - there goes any profit...
I make a bit of money on the side doing odd jobs - welding something back together for someone, clearing snow with the Bobcat, light woodworking, light repairs, that sort of thing. By the way, "quick" Bobcat jobs are likely my most profitable. Just something like clearing snow when you have six to eight inches or more. I can clear a driveway here in town in under ten minutes and they'd be out there for hours with a shovel or half an hour to forty-five minutes with a snowblower - if they have one. So compared to the work they'd have to do, $30 to $40 seems cheap to them and I'm making over $100 an hour... Likewise, when they are looking at the time to use a shovel, wheelbarrow, and the subsequent chiropractor bill; an hour or two with the Bobcat seems cheap at any price.
Anyway, I don't see these things as a business and very rarely will give a "price" for my work. My first rule is that my next door neighbor never gets charged for anything other than materials. We're always borrowing tools from each other, lending a hand, etc. His help is worth far more than anything I could charge for an hourly rate. Other than that, I usually just tell people to give me what they feel is fair. Mind you, I only take on little jobs as I have enough of my own stuff that I can't find time to get to. Most of the time I actually get more than what I would have asked for and those times I feel like they were cheap, I just file that away and am "busy" the next time they need something done. It isn't worth my time to have to argue with someone over fees or to get involved in some large, drawn out project.
So think about what your goals are first and work from there.
Of course the