Water Heaters - Rheem 21VP40E only less heated water

I have a Rheem 21VP40E water heater. Heater is 8 years old but initially when we moved in this house 5 years back there was continuous flow of hot water and 3-4 people can take shower easily. There is a gradual reduce and now only one person can take shower. Two showers cannot run together and we have to wait 15 minutes before other person can take shower even in one bathroom.
I know few things
1) I have not done any maintenance. Can some one tell me what maintenance I need to do?
2) I have checked the temp setting which is little over medium. Can I put on high?
3) What are regular maintenance things I need to do for water heater.
4) Someone said there will be some rust/mildew formed inside the water heater which needs to be cleaned. Is this true and how do I clean it.?
Thanks very much for your help.
Regards,
S2KFixIt.

3 or 4 showers in a row seems like a tall order for a little 40 gallon heater. This size heater will only give you about 26 gallons of hot water. Perhaps when you moved in the thermostat was set way up, and recently someone reduced the setting. It's also possible the tank is filling up with minerals thereby increasing recovery time and reducing tank capacity.

The first two threads give excellent info on maintenance.