Charge the battery, test the voltage -- you want over 12.4 volts for a good battery fully charged.
Bike is in neutral, on the side stand.
Switch is off for now.
Take jumper cables from a 12 volt car battery and ground the negative jumper cable clamp to the bike's frame on a plated piece of metal like the sidestand, pull back the rubber boot from the starter motor large red junction block and put a piece of metal that you don't mind getting scarred up into the jaws of the positive jumper cable clamp and just touch the exposed positive cable junction on the starter motor itself with the positive contact piece of metal.
Expect some sparks, some heat and some snap sounds -- DC carries some energy and the starter motor draws some decent power.
If the starter motor even tries to run, stop at once. Starter is likely not the issue.
Follow the big cable from the starter block you just jumped to back to the starter solenoid. Make sure connections are clean and not corroded. Check the large connection on the other side and trace the big cable back to the battery -- once again clean, no corrosion, nice and tight.
I generally make up a nice long fairly substantial red lead with an alligator clip on it to use for troubleshooting bike electrics. Put the clip on the positive on the battery and touch the exposed bare wire on the other end to the red small lead going to the solenoid, energizing it briefly as if it had just gotten a "go" signal from the starter switch.
If the solenoid clicks and the starter motor tries to run stop at once -- the solenoid is likely not the problem.
Now you are off into the electrics and the various safety switches and the handlebar housings and the starter switch up on the handle bars and the kill switch and all the rest of the convoluted wire runs and other crap that you will now have to deal with.
We have some electrical engineer types here on the list who are better troubleshooting that mare's nest than I am -- they can take over now.
If you don't own a volt ohm meter -- sing out now. You will be helpless without one.