2000 LT 80 won't start
 

I rebuilt this atv I got in pieces. It's finally done and I went to start it and I just heard a buzz or a hum at the battery, nothing else. The battery is charged.
Is there a fuse in the lines somewhere?
How to check if the starter is working?
Where do I start?

You can jump the starter directly from a battery to check function. Use a wire to ground the neg terminal to frame, and another to run from pos to the starter. Usually just touch it to where the battery already connects to the starter. Thicker gauge wire is best. Thinner will work for the short time you need to check function, but you really need at least 10 gauge so it doesn't heat up right quick.

Sounds like a relay buzzing which may be the problem.
There is a fuse thing under the right side rear fender (push the red dot).

The starter works fine. I pushed the red dot fuse, nothing changed. I swapped out the 2 wire white relay next to the battery, nothing happened. There is a 4wire black relay next to it. Could that be the problem or the main relay above the spark plug ?

Yep it is about $27 for the relay try tapping it a couple of times sometimes it free what's hung up in it. But it is a simple fix.