How do i turn off the hot water so i can fix a leaking hot tap
In an ideal world, there will be an isolating valve (see below) under the sink which you just turn.


Given you are asking the question, there probably isn't.

In that case you need to follow the pipework back until you find a valve.  Places to look:


If you have a hot water tank, there may be a valve to prevent any water coming out of it.  If there is, you can use that.
(If you have a hot water tank, and have to use any of the following valves, you may have to drain the hot water tank).  Check if there is a valve to stop water flowing in to the hot water tank.
Check if there is a valve before or after the boiler.
If you have a cold water tank, there may be a valve to stop water coming out of it.
Ultimately, you may have to turn off all water to the house (and if you have a cold water tank, you will have to drain it).  You do know where the main water stop-cock is don't you?  If not, now is a really good time to find it!  (Not when there is water pouring through a ceiling.)


Depending on how far back you have to go in order to cut off the water, you might want to consider taking this opportunity to add some additional isolating valves - it's very easy.