Dripping shower head and faucet

The faucet and shower head in my bath have been dropping for quite a while. Now, I can’t cause them to do anything but drip, no matter how high I turn the taps. What sort of work would be required in fixing this? Is it possible water has been leaking behind the wall and in that location could be mold? It would be highly appreciated if anyone aware of the minimum and maximum cost estimates for dealing a plumbing issue (http://www.greencityplumber.ca/residential/general-repairs/). Also, my hot water heater has been an issue for a very long time. It only stays hot for extremely quick showers, then goes cold. Also, I can’t do many dishes at one time because the hot water evaporates. How much might it cost to hold a new hot water heater put in? Thanks so much for any help provided.