Question 1: In the US, you can't. You'll need a parent or guardian to be the official business owner. Some paperwork you'll need to file will include an assumed name, articles of incorporation, and applications for federal and state tax ids. Most of this information is free online at your state's Secretary of State website.
Question 2: Be careful on fixed bids. You'd be surprised how many people think the scope includes more. An example: I recently had a client that asked for a login form. My specs said I would deliver a login form that accepted a username and password, checked them against a database, and if there was a match, it would log the user in. However, once the page was live, spammers came by and tried to brute force it. The client wanted me to install CAPTCHA for free. Not a big deal, but it was the straw that broke the camels back as it were, and I said it would take an extra half hour to do. They demanded it be done within the scope. It's a perfect example of the problem with fixed bids. As far as pricing, site down and figure out what you want your rate to be. Starting rates in the US are $20 - $40 / hour. If you do fixed bid, tally up the amount of time you expect things to take, then add a "cushion" to cover the inevitable out-of-scope-but-the-client-thinks-are-in-scope tasks. 10 - 25%
Question 3: I know some developers that work that way: 100% at the end. I know others that work 100% up front. The way I look at it is to spread risk equally. If I wait to invoice / bill at the end, I risk all of the money never getting paid for me doing 100% of the work. If I bill everything up front, the client risks me walking away without completing the project (not that I'd do that). So I set up my fixed bid contracts on a payment plan (2 to 6+ months depending on the size). That way, if the client stops paying, I stop working and I'm not out all the money.