OK I am an accountant, lemme correct a few things.
First off, you do NOT need a federal tax ID number unless you incorporate or have employees. If you do need an ID number, it does not cost anything to get one.
You may need a local business license. That is not a tax ID number. The cost of that license will vary widely, and may subject you to local license taxes (often based on receipts) or local property taxes on your equipment.
You may also need to register a fictitious business name (DBA) at the local level. That is a one-time cost for a filing fee.
sixways was basically correct about the home office deduction, but there is one giant big caveat: you can't use a home office deduction to generate a net loss from the activity. Therefore, you won't be able to offset other sources of income with a home office deduction, just your studio income.
Also you cannot simply writeoff a portion of you "house payment" unless you are renting. If you own, then you claim a portion of mortgage interest (which you are probably already deducting anyway), as well as depreciation on your house, taxes, utilities, and insurance.
The flip side of that is you have to pay the employer's half of FICA tax, which is 7.625% of your wages in extra taxes. That's what "self employment" tax is.
Strictly speaking, self-employment tax is both halves, or 15.3%, or 92.35% of your net, and you get a deduction against your income for 1/2 the self-employment tax. It works out to something like 12%.
The deduction for mileage last year was $.40 a mile prior to August 31st, $.48 a mile after.
It was $.405 until August, and $.485 through December, but for 2006 it's dropped to $.445.
You will also need to start making quarterly estimated payments to the IRS to avoid fees and penalties throughout the year.
Yup. Welcome the Working Week!