Expenses that are allowable for your business are expenses that are ordinary and necessary. You should be keeping track of all ordinary and necessary expenses for your line of work. Some common expenses are Advertising, Bank fees, Interest, Rent, Insurance, Travel, Communications (internet, phone, etc), Wages, Payroll Taxes, Contract Labor, Office expenses, Accounting, Legal, etc. Here's a link to the IRS answer:
https://www.irs.gov/pub/irs-pdf/p535.pdf