Welcome to the hobby!
Water: What you do to it depends on the water. Is it municipal water or private well water? If it is municipal water, then you will need to add a dechlorinator whenever you add water to the tank. If you are on well water, you might still want to use a dechlorinator that neutralizes heavy metals if your well has a high metal content.
Dechlorinator: There are lots of dechlorinators on the market. Which one is the best for you depends on if your water has chlorine or choramine. The web site for your town ought to say what they sanitize your water with. Seachem Prime is generally considered a good dechlorinator for tanks with axolotls.
Aside from removing chorine/chloramine, most tap water is just fine right out of the tap for axolotls. They prefer harder water, so some people harden their water in various natural ways, such as crushed coral. (You need to contain the crushed coral some how so the axolotls don't eat it.) If your water is naturally soft or you have a water softener, you may wish to do this.
I am guessing by your question that this is the first time you've kept aquatic creatures? Have you read up on cycling your tank? If the axolotls are not on their way to you yet, I recommend doing a "fishless cycle" on your tank.