If you're good at something, never do it for free. Honestly, I used to offer plenty of my services for free, or I'd undercharge, and it is probably my own fault for letting it happen too often, but too many people took advantage of me and my services, and I got burned out and uninterested. So I charge everything for everything I do because aside from enjoying what we do, money keeps us motivated to be interested in this person or company's problems. The days of freebies from me are gone.
So don't sweat it.. you need to be upfront about your worth per hour or per session. If they want your full attention, they'll pay for it.
So answering your questions:
You have to figure out how you charge. Per hour? Per session? Sending out an invoice? Accepting payment via Paypal? Establish your prices in the beginning and appear as a professional right from the start.
For students, I suggest offering a "student discount" which might knock $5 off your hourly rate, so if you are charging $25/hr, you can get the total and discount it by 10% or whatever.
While having a lawyer is always best, you can get by with some legal jargon on a website or in your document, but make sure YOU understand it. Keep it simple and state that you are providing services and your customer is responsible for making payments NET X days.
You make your own price and your own value. Be reasonable. For example, for a company, you might be able to charge them $30/hr+, but helping an individual, you may have to charge a bit less. But this is up to you, of course.
Look at your competition and similar salaries and figure out what you are willing to accept as your salary.