Many sink repair jobs don't require a plumber as long as the problem exists at the sink itself and not in the incoming water line. For example, unclogging pipes can be done with drain cleaner and a wrench. You just need to shut the water supply off beforehand.
To repair or replace a pipe, access the space under the sink and turn off the water by twisting the valve clockwise until there is resistance. Then, use a socket wrench to remove the fasteners for the P-trap. This is where many solid blockages occur. You can also use a plunger to unclog drains, provided there is a layer of water to seal. Drain augers can be used for stubborn clogs.