Charge it to full battery (led on the magsafe port goes green), once full leave it plugged into the adapter and turned on for about 2 hours (as in fully charged *then* 2 extra hours on the charger), you may use it in this time.
After its been fully charged and left with the power plugged in for 2 hours, unplug the power and leave it turned on, you may keep using it. Let it drain the battery all the way down so that it puts itself to sleep. To help with this it may be a good idea to change the energy saving settings to stop it putting itself to sleep after specific duration. You need it to put itself to sleep because it has no power left. Now that the battery is dead, leave it without charging for at least 5 hours.
After this, fully charge it and your good to go.
You should do this occasionally, like every 2 or 3 months, don't do it too regularly though.
EDIT: or as said above, read the manual and it'll say everything i just said in human readable form 😀