I would not put a lot of worry about a business license, if you're charging $20 you're probably not making the kind of money you need to worry about if you get sued. Photography is not a licensed business in most states in the US, maybe some have licensing but I doubt it. You should just charge what makes you happy, if you're not looking to do it for a full time business I think you'll be just fine. Now if you are sitting on a small fortune than disregard my advice. If you have something to protect file an LLC and that should be enough. I don't really know what a business certificate is, I'm sure you could create one in photoshop though that would do the same thing as anything that is paid for. Insurance for a business protects your business, do you have a business? So I wouldn't worry about the insurance, again if you get sued and have nothing you are paying insurance that won't do much for you. If you are thinking you can get enough information to start a real business on a forum than I suggest you take some business classes, but I think you're just shooting some friends photos because you enjoy it.
Sorry Shooter, but that has to be some of the worst advice I've ever seen given. I assume (perhaps incorrectly) that if the OP is taking college courses he is over the age of majority, and therefore if he's going to be collecting money for rendering a service, he's in business. Suggesting that someone forge a certificate... well, why not just suggest that he download a copy of Photoshop from a Torrent site too? Insurace is very important, critical in fact. People can and will sue for almost anything. If the person rendering a service doesn't protect himself, he's going to suffer at some point.
There's a world of difference between a elementary school lemonade stand and an adult offering services for a fee. IF the OP wants experience, my suggestion would be to do for free and ask the client(s) to give him a CD/DVD onto which he will burn the images when complete. That should avoid most if not all of the issues of concern.