If the bike isn't moving, ALWAYS let the clutch out in neutral before engaging 1st, especially just after starting. It should clunk into gear. If there's no clunk it will probably pop out of first. If it hasn't clunked, just work the clutch out a little and press it into 1st again. It should clunk. If you're moving when you engage 1st, it should be OK.
If it's popping out of 2nd or other gears it could be the detent spring. Do not let this go unchecked for long as it could be catastrophic. Relatively simple(ish) fix which can be done without removing the engine, so don't let your dealer/repairer tell you that they have to do so.