I watch others riding a horse it didnt look like fun going up and down it should be hard on the sit down. what can you tell me?

What you describe is called riding a posting trot - posting can help you avoid bumping on the horse's back. When learning to ride this can be a challenge but once you have learned how to ride the posting trot it is not hard on your bottom or the horse's back.

check out some of the articles on the horse's site and you will find more answers to all sort of questions.

Start with:

www.bellaonline.com/articles/art31841.asp
and

www.bellaonline.com/articles/art63752.asp