I have a 98 Grand Prix that won't crank over. I've changed the Starter and made sure the battery is charge with no low cells. Please help me!

I recommend you get the battery, load tested, perhaps at an auto parts place. A battery can read 12.75 volts fully charged, but no longer have the amperage to crank an engine. Or, come to think of it, have you tried to have the battery jump started? If it does start, then your battery is too weak no matter what the voltage reads