How can I convert a crawlspace to a basement?
Where do you live? If the plan is to use the space as living space, most code will require a minimum ceiling height. That's usually 7'. So you either have to accept that you can only use a portion of the crawlspace, or you have to dig down deeper.

To dig down deeper, you need to bring in a structural engineer to figure out how to underpin the existing footers. That's typically done with a buttress wall a few feet in from the existing footer. So you will loose some perimeter space due to that. 

If you have HVAC and mechanicals everywhere, keep in mind the time and costs to re-route all of that as well.

Enclosing a crawlspace is a good idea, but if the goal is ultimately to mainly get more livable space, you may want to investigate an above-ground addition instead. It may ultimately be easier (and cheaper) to do that rather than convert a crawlspace.