First remove the battery from the bike. Remove the seat and you should be able to locate the battery. Unscrew the positive lead wire, then the negative one prior to removal of the battery.
The battery is filled with electrolyte, and this electrolyte must be replenished. This is done by buying Distilled Water at Walmart.... Remove the caps on top of the battery, being careful not to splash any of the acid on you. Get a syringe or a bendable cup that will allow you to replace the lost electrolyte with Distilled Water.
Fill each chamber of the battery with distilled water up to the fill line. Be careful not to overfill. If you do, remove some fluid with a bulb syringe..again, careful not to spill any on you..that's battery acid!!
Get a trickle charger or a regular battery charger, they sell them at Wal Mart and they have different amperage/voltage settings on them. Hook the battery up to the charger and wait for the battery to charge. It takes hours in some cases, depending on the condition of your battery. You will know it's charged because the electrolyte will be bubbling when it's charged.
If you see/hear bubbles in the chambers, remove the charger and don't let the pos and neg ends touch. A spark could cause an explosion of the gases given off during the charging process.
Replace the caps or covers to the cells, put the battery back on, and fire the bike up.