axolotls vs. waterdogs vs. tiger salamander larvae
you definitely need to figure out which it is. LPS are usually pretty clueless as to what they're really selling. axolotls, like oriongirl said, are neotonic and should remain in their larval state their entire lives and actually breed in that state (that's like tadpoles of a frog remaining tadpoles their entires lives and breeding in that state). i actually have a pair of axolotls that morphed--which is extremely rare. true axolotls really aren't supposed to morph except for hormones being introduced from other tiger sals or bad water quality or other situations like those (mine was because of hormones introduced by other tigers...).
tiger salamander larvae can get pretty big in their larval state and might not morph for awhile--but they eventually will, and then they'll need a completely terrestrial setup. they actually look quite a bit like axolotls, but they are different. john claire has some sites that are pretty helpful in distinguishing what you have. do a search for axolotls or tiger salamanders or john claire on google and you should find some good stuff.
waterdogs are techinically a different species, which actually looks quite a bit different. they are completely aquatic (as are axolotls) so they don't require any land. you could fill up a 10 g to the top (just like a fish tank) and have a regular filter and everything and that'd be fine. you should pry create caves out of rocks or whatever to give them places to hide, tho. make sure it's sturdy!
the setup should be pretty simple for eithe rone of these, unless you have a true larval tiger salamander. in that case, you should fill up the tank halfway and have a big rock or some type of land thing that the "waterdog" can climb onto when it's beginning to morph. if that happens, then you'll need to be informed on how to deal with that (go to a kingsnake.com forum and ask them about it, or do research from google or something). but with the fully aquatic setup, you could use a regular back filter or even a sponge filter...but fluvals work pretty well. you don't need to heat because room temp temps are fine. you should feed them bloodworms, beefheart, or the like...but not too much brine shrimp, cuz they're really salty.
anyway, you should definitely do more research or post a pic of what you have. i actually bought my two axolotls on impulse and had no idea what they were or how to take care of them. 5 years later, i'm glad i did it

but i've definitely learned my lesson. keep me posted on how stuff goes and what they really are.
-chip