For federal purposes sole proprietors file Schedule C-EZ, Profit or loss from Business with their personal tax return Form 1040. Sole proprietor farmers file Schedule F, Profit or Loss from Farming. If you are a sole proprietor in Maryland you would file the same as an individual using Form 502. Your net profit or less is combined on the return with your other income and deductions and taxed using individual tax rates.
Sole proprietors generally do not have taxes withheld from their income so they usually make quarterly estimated tax payments. The Maryland form to submit quarterly estimated tax payments is Form 502D.
Filing information for sole proprietors may be found under Sole Proprietorships or in the Filing Information section under Individuals Income Tax section. For federal information on sole proprietors see IRS web site.