Ehow.com has an article on how to replace a car battery. There are also several videos on YouTube that you can find on how to replace a car battery if you are so determined to do it yourself. I would suggest have someone with experience put one in.

Battery life depends on many factors, but usually 3-4 years. 1 price paid for the battery, the higher the price the longer it should last. there are 36, 60, 72 grades on batteries, the higher the number the better it is. 2. conditions, extreme cold or hot will lessen a battery's life 3. usage, short drives where it can't be recharged completely, accessories running when engine is off will do this also. Lead/acid batteries have only a certain number of times they can be discharges and recharged before they wear out, if you want your battery to last longer you should pay a little more for it, and use the warranty if it does not live up to it's life expectancy.

Your Sirius car kit should have come with installation instructions. If it didn't you can also find step by step instructions here, http://www.fixya.com/Sirius How To Install.