How do I get hot water in my shower?

Hi,
My shower doesn't seem to produce hot water anymore. When you turn on the immersion switch it only has a very dim light and then when you turn the shower on the light goes out completely!!! Can you advise me on what I can do to replace or repair the fault myself as can't afford a plumber / electrician!!!
I'm not sure of the model of the shower!!
Thanks,
Gav