To move the piston in, use a vice grips or channel locks and the old pad, grip against the caliper, and squeeze the pad in toward the caliper. It may make it easier to pop the cap on your break fluid reservoir (to allow the pressure to get out).

Other people have suggested using a thick heavy screwdriver and using it as a lever to push the piston and pad back in, but I have never done it that way, so I don't quite know how that goes.

If it is just one pad worn, either someone did an incomplete brake job (not very likely?), or you may have a piston that is sticking.

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