I have low pressure at the hot tap. How can I fix this?

Low hot water flow and pressure at the tap could be low incoming water pressure, a faulty thermostatic valve in the tap or shower or a fault combination / pressure reduction valve.

Has the water flow and pressure always been low? It might be an external problem
Is the problem at just one outlet e.g. the shower. Probably just a shower valve fault
Has the problem just started and is slowly getting worse, and it affects all the outlets? Probably a pressure reduction valve fault