1. Are you choking it? If it's a cold engine, you will need to.
2. Is the spark plug wet or dry?
3. Is the compression OK?
4. Have you tried priming it? Just squirt a small amount of petrol in through the spark plug hole immediately before trying to start it. (Don't use the choke.) If that makes it fire, run briefly, then stop, fuel is not moving from the carburetor to the intake pipe. We can talk about what causes that, if it turns out to be what is happening.
