If you are sure there is gas in the carbs the next step is to see if there are sparks being generated.
Grab a metal wire and insert inside the plug that connects to the spark plugs. Hold the other end of the metal wire close to the engine block. When you start the engine you'll see a small spark.
Sparks ok: you probably tried to start too many times and the spark plugs are wet. If so, remove them or wait for them to dry out.
Not much else. Gas + sparks is what you are after.
Some tricks: hold half throttle while starting the engine. Use the choke. Hehe