I have never read the books, I've only seen the first two movies. I've wondered about what the imprinting part that jacob had to do and why it had to happen?

He didn't have to do it.

It's complicated to explain. It would be a lot easier for u to read this page:
link

Imprinting is not something that u have to do it just happens out of nowhere.It's a connection and a feeling that a wolf has that cannot be controlled. Some wolves imprint and some don't.
Jacob went to kill the baby when the imprinting happened. I suggest u watch all the films to understand.

Basically, it happened in order to close off the love driehoek fully, and to give Jacob a happy ending instead of having him mourning Bella for years. It also created peace between the wolves and the Cullens, and made it impossible for the wolves not to be fighting alongside them at the end.