Disconnect the battery cables, clean the cables and the battery posts, reconnect. Then turn the key and see if there is a difference. If not....
With a multimeter check the battery. You're looking for 12.4-12.8v, leaving the black multimeter wire on the ground post of the battery put the red lead on the stud on the starter that the battery connects to the reading should be the same that you got on top of the battery. Move the red wire to the fuse block on top of the battery, again you should have whatever reading you got on the battery on each side of each of those fuses.
If any of the reading are 0v you've found the place to start looking for a bad connection.
Cheap digital multimeters are available from Walmart or Home Depot for less than $30. Go on youtube and learn how to operate one if you don't know how.