sometimes the clutch will stick after sitting for a few days - certainly if it sat for a month
you start it up in neutral and when you pull the clutch in and kick it into gear the bike jumps forward. Its because the plates are in the oil, and after sitting for a while the film is pressed very thin between the plates, and they stick together.
If the bike is jumping when you kick it into gear, that is whats happening. Try giving the bike a bit of a push with your feet to get it moving then kick it into gear with the clutch pulled in. If its really stuck you may have to pump the clutch lever and get on the gas a bit to break it loose while the bike is moving.
If you get the bike moving and then kick it into gear, and its still stalling, then something else is going on.
How are you dealing with the choke?
all the way out to start when cold
push half way in when it starts
push it the rest of the way in about half a mile down the road