Im guessing your young, and this is your first time. I know a few people that had that problem when they first started having oral or sex. Like everyone else has said, jerk off more, and while your taking part in the fun try and think about anything besides whats going on at your waist. Personally I never had this problem my problem was getting hard and staying hard I was so nervous about what was getting ready to go down I just couldn't keep him up the first time.