Obviously a wire that is shorted to ground or two wires shorted together, less likely but possible is a faulty turn signal switch/flasher or even bulb casing. Does the fuse blow regardless of left or right signaling? Does the fuse blow with the tail lights pulled out?
Electrical troubleshooting is best done using what is called the half-split method. For example, keep dividing the possible trouble area in half by ruling out say, left vs. right tail light. Then maybe before or after the flasher unit. After that, along the first half of a length of wire or the second half, etc. keep disconnecting components of the lighting path until the fuse stops blowing. You may need a supply of fuses for this method.
The best way, of course, is to use a continuity checker/volt meter to ascertain when the short disappears as you disconnect various components (ignition off). My guess is considering the way you asked for help, this method might require a skill you don't yet have.
I would start by pulling out the tail light bulbs and measuring across the sockets and look for a short there. If found, work your way up the wire to the flasher and switch, ruling each one out as you go.