Fly? Pretend to be Casper and be ah cool and nice to people and try to make friends with other ghosts or annoy/scare the fuck out of people.
go into a girls house and hold up floating panties imagine seeing your underwear floating as you sleep.
It's hard to spook people where they are in an excited state. You'll need to be able to throw stuff around to get their attention.
How can ghosts have sex? *realises she knows nothing about ghosts and should probably leave this thread...*
You can't. When people make love there auras converge and create a temporary force field separating them from the spiritual omniverse. Only material objects can penetrate it.