Need more info. Which engine. (3.5 Briggs?) How long have you had the mower? Did it run good last year? Any recent work? Old gas still in tank?
But anyhoo, here's a few things to check. Spark plug. Clean, new plug with proper gap. Clean air filter. Choke working OK? Last years/old gas still in tank? (Gas doesn't keep.) Hang in there, you'll get it.