You have to be sure the battery has a full charge or you can't do any testing. Check the charging system also.
If you have an excessive drain with everything turned off. Check for a lite staying on, best time to check is in the dark, trunk--underhood-- interior. You can take battery cable loose, use a testlite between cable and battery post, if lite come on birght, you have a drain. If the drain is on a fused circuit, you can pull/replace fuses one at a time, when you pull the fuse on the faulty circuit, the lite goes out.