You deffinately need a business plan of some sort.
I'd recommend taking a small business course at a local college.
You need to break down all of your expenses so you have a ratio of expense to income.
This will give you an idea of how much to charge but always keep in mind that some things that can earn you income are not always something you can record at the end of the month books. ie, lowering costs for a customer that could potentially bring you in more business.
Do you have a business liscense?
Do you know what type of business liscence you need?
I found out that just having a business liscence wasn't enough if I did work in a town that was different than where my business address was located.
Also, be prepared for a headache at taxtime...and know when your business tax is due.
Anyways, this is just a small fraction of the knowledge you need BEFORE you get involved with your business.
Otherwise, you'll be looking at failure, dissappointment and maybe even legal problems.
You can do it. You just have to arm yourself with as much knowledge as possible.