Faulty bath tap?

[skt1977]

I recently bought this flat and moved in. I found the bath tap is not working i.e. the overhead shower faucet works but the tap is dead. I am not sure what part here needs replacement, I suspect the knob needs to be pushed down and probably rotated for the tap to work but it just does not happen.

I am not in to plumbing at all, though I understand the distinction between what can be done and where you need to seek pro help. Can I fix it myself ? If yes what needs to be bought and what precautions before starting ?

Thanks
SKT