What kind of ancient Greek dialect is usually learnt?

In refutation of Jose Pineda:

• You need Old Ionic (Epic) to understand Homer, and all of Greek literature is suffused with Homer.
• You need Ionic for Herodotus and Hippocrates, and the authors imitating them (more of them for Hippocrates, for Herodotus just Lucian in one work).
• You need Doric for the choruses of the plays, as well as a lot of poets (not just Alcman of Sparta), and to know what’s going on in half of Aristophanes’ plays, where Doric-speakers show up.
• Aeolic you need for Sappho, Alcaeus, some poems in Theocritus—and to know how different Greek dialect can get.

Luckily, you don’t need to know much dialect for most things you’re likely to read in the canon—the Doric of the plays is quite superficial, and there’s not much variation in vocabulary.

But to go back to your original question details:

every time I look up a word in the dictionary, I find like seven versions of the same words in different dialects. Which one am I supposed to learn?

Liddell–Scott is an historical dictionary, so it tends to give Epic first, as the oldest form. If you’re learning ancient Greek, the form you learn out of the options given to you in an Ancient Greek dictionary is the Attic one: it is the dialect of most of the canon.

But you should be aware of the derivation of the form—particularly the uncontracted forms—so that the Attic form makes more sense in context. And being aware of the Epic form is no waste of time.