I would hate to think that you could possibly be pregnant when you don't even know how to figure out when your next period is. On a 28 day cycle, which is 4 weeks, your period should start on March 31. For a pregnancy test, you would have to wait 10 to 14 days after missing your period. That would make it around April 10-13.
Some of the newer pregnancy tests claim to give you an accurate reading as early as five days after your missed period. Check your local drug store you could get your answer as early as today. Good Luck.