I recently discovered that my full size 1999 chevy express van comes with keyless entry. I brought a transmitter and i want to program it myself can someone help

Welcome to AF.
Jumper no 4 and 8 pin on your diagnostic connector.
Doors should lock and unlock.
If so on your remote press the lock and unlock button at the same time.
Doors should lock and unlock.
If any more transmitters do the same thing with them.
Remove jumper wire and you are set to go.

If the transmitter ever needs reset.
Press and hold the lock and unlock buttons at the same time for 7 seconds.