there is probably a bad seat or aquaseal washer on your shower stem that has worn out

find what brand of handles you have
remove the handles off the stems
there is probably a threaded nut over the stem holding it in(unscrew this)
the stem should pull out or unscrew

there may be a seat behind this that should unscrew with a hex-head insert or allen wrench
check the seat if a groove has been worn in it

replace the seat
replace the stem or install new aquaseal washers(the rubber top on the stem
reassemble

you should be able to get parts at your local home improvement store or plumbing wholesaler

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