To keep your own books, consider using a check-register type computer program such as Quicken Home & Business or Quickbooks (by Intuit) to track your expenses. If you are doing your own tax return, use the companion program, TurboTax.
To make sure you're on the right track, it's a good idea to run your bookkeeping system by a savvy, small business tax pro. With just a few hours of work, he or she should help you avoid most common mistakes and show you how to dovetail your bookkeeping system with tax filing requirements.
When your business is firmly in the black, consider hiring a bookkeeper to take care of your day-to-day payables and receivables, and an outside tax pro to handle your heavier-duty tax work. Not only are a tax pro's fees a tax-deductible business expense, but chances are your business will benefit if you put more of your time into running it and less into completing routine paperwork.
For help on finding the right tax professional for your business, see Nolo's article How to Hire a Tax Professional for Your Small Business.