Im looking to store maybe 10 gallons of gas and 20 gallons of diesel in regular containers in my garage.

How long will it last and what do I need to do to it?

For long term storage of fuel you need to add an additive. One of the better ones is a product called Sta-bil.
http://www.goldeagle.com/sta-bil/index.htm
In theory is you treat your fuel at the recommended intervals it will last indefinately.

Make sure its sealed.

I wouldn't count on the fuel being able to stored indefinitely, If you can rotate (Use and replenish) aim for a year.

Try and get a filter as well.

It will be stored in a separate garage. I couldnt care less how much I'm allowed to have...

Does Sta-bil go in the diesel too or just the gas?

A filter for what?

I believe that there is a Stabil Diesel version

Stored diesel will gum and create particulates. Both can stop the flow of fuel to the motor. See - http://www.catalina36.org/articles/recalls/dieselfuelstorage.pdf

If the engine you are feeding has a inline filter you’re ok (Having a spare or 2 is never a bad idea). If not, you’d want to filter it before putting it into the engine reservoir. I would think that TSC/Fleet Farm/Northern Tool would have one.

Ahh, I understand.....

The diesel would be for my truck...

I would probably rotate it out every month...??

Is that short enough to not worry about it to much?

As long as it is well sealed in an appropriate container you should be fine.

"It will be stored in a separate garage. I couldnt care less how much I'm allowed to have..."

That might not be your opinion when you are in prison becuse the garage exploded and killed someone, like say your spouse or children. It's illegal to store more than a few gallons of petrol in the UK. Check if it is legal where you are.
The Sock

Diesel’s primary enemy is water. Dino-diesel is free of water when distilled. Bio-diesel has water in it when it is created and the water must be completely removed before it is sold if it is to meet current fuel standards. The old style low pressure mechanical or electrical diesel injectors could survive some water for awhile. The newer diesel engines with high pressure common rail fuel systems are extremely harmed by any water. When storing diesel fuel, completely fill the container to prevent water from condensing on the container’s interior surface and getting into the fuel. If you have a way of draining the bottom of the fuel storage tank, add a non-alcohol based demulsifier and drain the water from the bottom of the tank before use. Opti-Lube XPD, Opti-Lube Summer Blend, or FPPF RV, Bus, SUV, Diesel Treatment are three of the best additives to use. These treatments also add lubricating properties to the new Ultra Low Sulfer Fuel now mandated in the US. Most diesel equipment operators with the newer high fuel pressure systems do not use diesel fuel more than six months old. After six months, the fuel is tested, filtered, and retreated at the least.