How do I lock the tailgate with the keys? I can't seem to figure that out, lol.

1993 s10 Blazer.

 

the glass should only be able to be opened from the inside

 

Means - no need to lock it, you can't.
The handle should turn only with the key inserted. Once you remove the key it is actually locked as you can no longer turn the handle in either direction (left to release the glass).