That plays after the opening credits in the first 2 or 3 season of Happy Days, once you watch one those peisodes you know which one I am tlaking about, I really want to know what song that is? Please tell me

I think you're referring to "Rock Around The Clock" by Bill Haley And The Comets. (1955)

I know that one, but after the opening credtis, like when the episode is starts a scene ther is some knid of instrumental song or something playing durign 6the first scene in the episdoe

Hmmm.
Don't know that on off the top of my head.
I'll have to get back to you on that one.

I bet it's the same melody that begun later Happy Day episodes beginning in the 3rd season. Perhaps there was a different arrangement or something. I remember it from the end of quite a few early episodes.