It could be a number of things. Are you using a new battery?

Assuming the battery is OK, is the starter OK (it turning the engine over?
If yes, I'd check:
1. Ensure you have spark
2. Ensure you have fuel delivery

If you've tried a few times and had no luck, you might have flooded the engine. In which case you'll need to pull your plugs out and air the cylinders (if that hasn't already be done).

But yeah, first check for spark, and fuel delivery and take it from there. Post back if you're not sure how to check for spark.