If you have an electric water heater, the power must be switched off first.
When refilling, I open up a tub faucet to bleed air out of the tank. Since there is no aerator on the tub, (most of the time) it's the best place to bleed air out.
Don't turn the power back on until the tank has completely filled with water.
If the cold water shutoff on the water heater is not sealing, that's a second job. The water will need to be turned off at the main, and if you are using flex connectors, throw away the old one and use a new one. I never reuse a flex connector.