This is what is called a parasitic draw. It describes that some electrical component is staying alive when the key is off and drawing from the battery.
You are going to need to find out what system is drawing down the battery. If you know how to use an ampmeter, you remove the negative cable from the battery, put one lead on the battery and the other lead on the cable. With the key off there should be no more than .03 amp draw. You can do the same thing with a test light. With eh key off the light should not be on at all.
If the draw is too large or the testlight is on, start removing fuses until the draw goes away- that will tell you what system is drawing the battery down.
Then once you know what system is the problem, you have to diagnose it further to find out exactly where the problem is.
Some common areas of battery parasitic draw are the glovebox light staying on, the truck light, the center console light, a sunvisor light. These re the simple things, but the problem could be much more technical.
