Identifying the breaker should not be a huge issue. Just plug a radio or vacuum (anything that makes noise) into the outlet and work your way down until you knock it out. Of course the downside is resetting clocks all over the place...
Once you have the power out, unscrew the receptacle and pull it forward. You'll see either one set of wires (black and white most likely) or two pairs, two black and one white.
Now for the GFI... when you purchase one, open the box at the store and make sure the back looks like the attached photo. Most, but not all, GFIs look like this... behind each screw terminal, there are two holes. That's the style you want. It makes the wiring much easier.
Notice that this GFI has one set of terminals covered by a label. More on that in a bit.
The existing outlet will have it's wires pushed into locking holes in the back, or wrapped around screw terminals. In either case, I'd recommend cutting the wires and stripping them again to expose new wire.
If you have one set of wires, simply slide the exposed wire into the appropriate hole (back for hot, white for neutral/white) and tighten the respective screw.
If you have two sets, it gets trickier. The first thing you need to do is make a decision. There are two ways you can wire it.
A) You can have just that outlet be GFI protected
B) You can have that outlet, and any outlets on the same circuit that are "further down the line" be protected.
If you want just that outlet protected, simply slide both black wires into the two holes provided (do NOT connect to the terminals under the label), and two whites into the two white/neutral holes, and tighten the screws.
If you want the other outlets protected, let me know and I'll peck out those instructions.
The only other step is to firmly connect the bare ground lead to the screw terminal provided on the top or bottom of the GFI, and install. Some GFIs can only be mounted one way up or they will not function properly. Check the metal "ears" as they may be labeled with "this end up" or something similar. If so, make sure you mount appropriately.