Sounds like the caliper has seized.
Normally the piston (inboard) presses against the disc, this causes the caliper to slide and pull the outboard pad against the other side of the disc.
If the caliper cannot slides on the carrier then just the inboard pad will push against the disc and wear very quickly.
The caliper slides on two large 'pins'.
If you remove the caliper (two large bolts) and lift it off the disc you should be able to slide the two parts of the caliper on those pins...you'll probably find it very difficult or impossible, hence the pad wear problem.
Remove the old pads and discard.
It should be fairly easy to fix.
If you can seperate the two halves of the caliper (slide the one half off the pins) you should give the pins a good clean - DO NOT use any abrasive papers!
They often get jammed up with crud (old brake dust) which can be rubbed off - a soft plastic scraper helps, some brake cleaner will also help to lift off the grime.
Don't forget to clean up the holes that the pins slide into, same process.
Give the pins a good coat of high temperature grease and slide the two halves of the caliper back together. Slide them about to make sure nothing sticks.
Push back the piston, by hand if you're lucky, use a g-clamp if its stiff.
Put some new pads in and away you go
Just bed them in carefully.
You must replace BOTH sets of front pads :!:
