this is the basics of fault finding .
Remove the plug, reconnect it to the HT lead, hold it on the cylinder whilst turning the engine over and you should see a spark.
Whilst plug is out do a compression test. use a decent compression gauge to do the test .
Check fuel is fresh and getting to the carb.
thoroughly clean the carb out
Check air filter is clean.
check timing spark problem - duff plug, cap, ht lead, coil - CDI , check timing .
Try the obvious first.