Okay, so:
You should have an LLC or S-Corp.
That LLC or S-Corp should have it's own savings account and checking account and also their own credit card.
All business expenses should be run through those two accounts. Deposit checks to the SAVINGS account only and transfer cash as necessary.
Buy Quickbooks/MS Money or set up a simple Excel spreadsheet.
For each deposit you need to denote what the deposit was (ie, income, refund, etc).
For each expense you need to categorize the expense (service, office supplies, etc).
At the end of the year take this all to a CPA (NOT just an accountant) and have him/her file the corporate return. If expenses > income you owe nothing.
If income > expenses - then the CPA will figure out what you owe.
Using a CPA gives you some plausible deniability if you are ever audited. There is always some grey area on what is/is not a valid deduction. If you used a CPA for your taxes however you can show you had a "good faith" attempt to get your taxes right and if you ever are audited you are probably going to be looking at some minor fines, etc.
Most of the time you do not get into "REAL" trouble with the IRS unless you hide income - and not declaring that $100 of adsense earnings would count as hiding income.