try tightening the nut around the stopcock (wall tap).
if still leak you my need to change it.. go hardware shop have 2 sizes, best you bring your own. make sure off the main. or just buy 2.
not easy to change btw.

if the leak is at stopcock is good news, just need to hack a bigger hole and change stopcock.
if leak else where you will need to do a bypass.. means pipe runs outside the wall instead of the original in the wall.

basic plumbing is about available parts. just go google stopcock parts or valve or find in e-shop for replacement.
professional plumbing is different level and takes experience.

my apartment had a leak in the wall, took 5 plumbers and few months, and working with building management to get it diagnosed correctly.
the default so called plumbers will say water proofing or bypass.
you listen you die.