Start by jumpstarting the bike with good battery - a car battery is fine. If it starts you know that a low battery is the cause of your initial problem. If it doesn't, you need to look at the solenoid and starter.
Assuming that it starts, measure the voltage across the battery terminals with a multoimeter. It should be between about 13.6 and 15V with the motor running at about 6K rpm.
If it's higher you have a u/s regulator that's frying your battery.
If it's lower you need to unplug the r/r from the stator and measure the ac voltage at the stator connector. Again at about 6K rpm.There are three windings which form three pairs measuring each to each. Not sure what the voltage should be, but i'st about 40 - 70 VAC. That's important. Set your meter to AC for this measurement. If one or more windings has a low reading you have a stator problem. If they're all OK but you have low voltage across the battery (after reconnecting the stator and still at 6K rpm) you have a faulty r/r.
If these checks are OK but the bike is still reluctant to start on its own battery, even after a ride, you probably need a new battery.