If your battery is good and your battery cable connections are clean and tight and you jumped the two posts on the solenoid and it didn't start, then your starter is likely bad.
You can do a quick test of the starter by using a set of jumper cables and a known good battery. Hook one end of the jumper cables to a good battery. At the other end of the cables, hook the ground clamp to the engine or starter housing and firmly touch the positive clamp to the starter terminal post. The starter should turn if you have good connections with your jumper cables. If it doesn't your starter is bad.
I like to use a battery in a vehicle that I drive every day, so that I know it's good and it's also easier to hook the jumper cables to an automotive battery.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.