alright i built one near my cottage. we climed way up a tree and tied the rope. the tree was ontop of a 15-20 foot cliff so if you can find a cliff to build it off, do that. as for ropes, the longer the rope the better anddefinently use a thick rope. we originally used a thinner rope and it sucked so we switched it to a thick rope and it was way easier to hang onto.
hopefully that helped.
when my friends and i built our rope swing we went into the hardware store and there were many different kinds. i suggest a think softer rope. make sure that you burn the ends so it does not fray.