Re: New to this
Howdy and welcome.
Is a very broad question, so no easy answer. Would check out the DIY section. You can make an engine move a bike really easily and what you want (good MPH or MPG etc.) is what you should look for. Sky is the limit, limited only by imagination. (and cash, snork)
A junk weed whacker will work or a $2,500 engine/gear reduction.