If it's from a piston seal, unless you're familiar with overhauling brake calipers (unlikely) I'd advise buying a reman caliper instead. They're about $150 from a dealer (OEM part) and you can replace just the one that's bad but check the other side (same axle) to be sure it's not leaking (rare), pistons move back easily and that the piston dust seals are in place (not popping out).
When you replace a caliper, you'll need to bleed the air out of it using either a vacuum bleeder or another person to pump the brake pedal (assuming that you know how to do that?).