Sounds like the bottom element is covered with mineral deposits.
Turn off power supply, allow to cool down. Turn off water supply. Drain water through drain valve (looks like a faucet) and garden hose to outside or drain area. (Open kitchen hot faucet to allow air in to tank to speed up draining.)
Remove bottom element, insert hose from wet/dry vac and suck out the deposits. (Using a wet/dry vac, not what you are probably thinking.)
This can be a messy job, plastic sheet or pans can be a nice thing to have.
Get out all the gunk you can, re-install element (may need a new rubber gasket), close water drain valve, open water supply, allow tank to fully fill (open hot water faucet in kitchen or bath to let air out), then turn power back on.
If it's a gas water heater, then Im not sure how to clean it, look for a drain plug on the bottom of the tank.