There is probably a little flap on the top, right where the seat is hinged, flip that up and you should see the top of the bolt, it will probably have a slot for a screwdriver (a BIG one). Underneath you will find either a wing nut, or just a nut. The bolt and nut should be brass or a white plastic, depending on how old the seat is, how much it cost, etc. If it is metal it will probably be frozen. In that case you can spray it with WD40, or my wifes secret, hair spray. Have fun. If a lot of guys have been using the toilet be prepared for some cleanup once you get the old seat off...