Go to the IRS.gov website and look at/print a copy of the Schedule C form. This will give you a good idea of the expense categories the IRS wants to businesses to use. This will help you set up a simple chart of accounts, like Jason recommended, to track your expenses throughout the year.
I would also encourage you to seek out a bookkeeper, NOT an accountant, to help get your receipts organized for tax time if you are not comfortable using software like Quickbooks yourself. A bookkeeper is often just as skilled at setting up your chart of accounts and educating you about the deductibility of expenses as an accountant for far less money. Accountants, by and large, do not like to deal with the mundane task of bookkeeping and will charge you dearly for their inconvenience.
As an analogy, it's like going to your doctor's office. The physician doesn't take your height/weight/blood presssure/blood sample (even though he/she is capable). The physician would rather use his/her expertise to diagnose your symptoms. An accountant would much rather spend time looking at your financial reports and advising you how to save money on taxes, whether or not to incorporate, plan for an expansion/budget, etc., than key 300 receipts for perishables, nonperishables and small equipment purchases.
Look for a certified and/or licensed bookkeeper (not simply someone who has taken a couple of accounting classes) and you can rest assured that your receipts will be handled appropriately. A bookkeeper typically charges $25 - $60/hr. A CPA typically charges $75 - $150/hr or more. Take the savings and reinvest in your business.