First, turn off the gas/electricity to the heater. Next, turn off the cold water inlet valve on top of the heater.
Make sure that the overflow pan under the heater is allowing the leakage to exit the building.
Next, contact Home Depot and tell them you want to buy a replacement water heater and have them install it. They have an arrangement with a plumbing company to do their installs, so you are looking for a package deal: Bring new unit, remove old unit, install new unit, haul off old unit. There will be a permit and inspection to insure that they did a job that meets code. Be sure to save the green inspection tag in case the city forgets to enter the inspection results in their database.
Water heater replacement units are typically bigger than what they replace (and more expensive as well) for the same water capacity. It has to do with fuel efficiency. There may be a little bit of sticker shock.
Glen is awesome. I got fed up because he wouldn't return my calls or texts and I wish I had kept trying to reach him (he is incredibly busy).
I ended up calling someone else who charged me $300 to not do anything. I had told the other plumber in advance what I needed done - he came to my house and walked around and said that his rates would be high for everything I wanted done or he didn't want to mess with it and my SO should fix it. I called him because my SO didn't have time! Then I had to write him a check for $300..I'm still mad.
Glen came out on a Sunday and his rates were too low. I hope he has raised them since then.
I had Glen's guy out this morning. No problem getting him out here. I called yesterday at 4:30 for an issue with an upstairs shower and he was out here at 9:30 this morning. I wasn't in a rush as the issue wasn't a huge deal but I am glad it's done. I thought the price was more than reasonable. I don't think this was something RB could have easily fixed (he is so busy anyways). Guy was done in 30 minutes. Glen also did our water heaters 2 years ago. He was so fast to get it done for us. Nice guy.
Just make sure installs follow new codes vs old equipment. If not, one may have some buyers remorse down the road. Our new water heater is now so hot that I have to turn it way down even with kids and so much better. Get what you pay for.