I have a Rheem 50 gal. Electric hot water heater and it's not heating my water up , what could be the problem and is it simple to fix ?

Most electric water heaters are pretty basic, so hopefully this won't to be hard to fix, or get fixed, okay? First you want to determine whether there is main power coming into the unit (240v). If not, check breaker. If main power in is good, next you will want to check the reset button located on the top thermostat. If that is not tripped, check for voltage to the top element (once again, should be 240v.) If you have voltage to the top element, and no hot water, odds are you need a new element. You can test both elements with an ohm or resistance meter. Here, you would kill power to the heater, unhook the wires from the elements and test the elements.
Basically you have one of three things wrong. First is no power. Second is bad element/s, third is bad thermostat/s. Testing these items requires a general knowledge of electricity, and a multi meter. Please let me know what you find.

Fastest check for NO HOT WATER is to remove the top access panel and push the red reset button (see image). If you hear a click sound when you push the button you will have hot water back again in 20-30 minutes, but most likely it will only be temporary. In this case, you'll need to replace the upper and the lower thermostats for best result.

You'll also want to check the upper and lower elements for continuity, but as Mygirlsdad said, this requires a basic understanding of electricity... ;)

Check the reset button...

Back to you...

Mark