In your case 99 times out of 100 the battery is toast.
Changing a battery ain't hard just follow the directions on how to fill it with acid. Bike batteries are generally shipped dry and the acid is in a seperate jug. Fill it to between the lines on the side of the battery and be patient, it takes awhile to fill the whole way and let all the air bubbles come out before you move to the next cell. Do not over fill it and keep the sulfuric acid away from your eyes and skin. Matter of fact, wear safety glasses. Then you'll want to trickle charge it on like a 2 amp setting on a charger for however long the instructions say to. Again RTFI (Read The Fricken Instructions). Keep it away from open flames or sparks while charging and don't smoke around it. Battery charging makes hydrogen gas, albeit small amounts but it can catch fire when exposed to flames or sparks.
I'm going to assume your battery is under the seat so you'll need to flip the seat up. Then you remove the rubber strap if yours has it. Next you remove the terminal leads positive side first. Pull the battery out of the box. Clean all gunk and stuff out of the box before puttin' the new battery on. If you see white flakey stuff in there mix up some baking soda and water to neutralize the acid. I would touch up any missin' paint while you have it out. Dispose of the old battery properly.
After you get all that done and the new battery is fully charged and ready to go put the vent hose on it. Route the vent hose while sliding the battery into the box, keep the vent hose away from the exaust, hot surfaces, and any chrome parts. Replace the terminal leads in the right spots, positive side first then the negative. Replace the rubber strap that holds it down or however it's held in there, if yours is missing get one. Put the seat down and try it out.