how 2 pull up codes on 1989 ram 3/4 ton 150

how do i go about pulling up the codes on this truck. its hard to start and seems to flood out. someone said it may be the oxygn sensor.
thnx

I have a 1988 D11, not sure if it's the same operation, but to check the codes for mine, I just turn the ignition to the 'on' position twice quickly and leave it on the second time. The check engine light will flash your codes. ex: flash flash (pause) flash=21. when you get the code 55, then it's through spittin' out your info.