Which verb form should I learn?

[squarezebra]

Pardon me for sticking my nose in on this one, but I'm with Sceptile in that I think its a better idea to learn the dictionary form from the off. And for a number of good reasons.
The majority of all study guides teach conversational Japanese, and as such provide you with the polite forms first, and that's fine if your goal is just communication i guess. But knowing the dictionary form + conjugation rules will provide you with far more flexibility when you start making more complicated sentences. Not to mention, the majority of dictionaries and webtools relay on you nailing the dictionary form:
If you find yourself a little unsure of how to conjugate check out this resource Japanese verb conjugator - you put the dictionary form of the work in and you get a complrehensive list of conjugations.
Unsure of what the dictionary form is? place the conjugated verb you have in here: Japanese verb deconjugator and it'll give you the dictionary form.
and of course, my favourite resource in thr world: if you want to check how these words you are conjugating/deconjugating are used in practice then head over to WWWJDIC: Word Search
type in your word in dictionary form click search, and then again on the [ex] box... voila ... its amazing.
If you're lucky enough to have an Iphone then buy CodeFromTokyo because that dictionary has all the common conjugations for verbs and adjectives built in .... that was a godsend when i first started out.