Buying our first home...from a private seller...HELP?!?

We are buying a home from a private seller, no realtor involved. Would it be wise to have a lawyer? We have been offered a loan and now need a purchase agreement. The seller has never sold a home and we've never bought. There seems to be so much to do..where does the purchase agreement come from? What else is it important to do? Is there anywhere we could get like a checklist or some help to know what we are doing? Thanks so much!