If you don't know what you're looking at, self diagnosis can be hard.
Do you know what the caliper looks like? I'll assume you don't, jut in case.
You need to get the wheel hot. Then remove it. As you look at the disc brake assy, you have the rotor, (the round disc behind the wheel). Depending on how the engineer designed it, the caliper is the lumpy looking thing about 2x the size of your fist. The pads ride in this, on each side of the disc. If it is leaking, you'll know it. Very carefully see how warm it is. Very carefully! It may be hot. Also, check to see if any fluid is leaking from it; the fluid will be a black-or-brownish clear. If there is leaking, the caliper is probably bad and needs to be replaced. If not, look inside the caliper 'clamp' at the pads. The pads may be worn down so far that the caliper has allowed the piston to slip out and lock on the front or rear, and the pad being pressured against the disc is causing it to heat up. If either leaking or locked, it needs to be replaced. This is actually fairly simple; there are two 10mm bolts on the rear of the caliper that hold it onto the hub assy. They *can* be rebuilt, but they are also ~$65 at AutoZone or CarQuest.
Also, have you had the brakes done recently? I did them all the way around on a Celica, and for about 10 days the rear wheel on one side was hot, but went away as the pads wore in.
Sorry if I was too simplistic, I don't know what your level of expertise is...
Again, not knowing how much you know, check this:
This is for brake rotor warpage, but gives a good description of the parts. On the caliper picture, see the bolts? That is the only thing holding the caliper to the assy. Undo those two bolts, remove the caliper, replace as required, etc. That's all there is, in a nutshell. I was afraid of doing brakes, but I had a nissan 240SXZ and no money for repairs, so I bought a manual. Discs are easy, drums are slightly harder...