/lit/ guide to learning ancient Greek

Which dialect do I start with??

Attic.

(alternatively, Homeric, through Pharr's Homeric Greek)

Which textbook should I use??

Most popular options (note I have personally no experience with all but one of these textbooks)

JACT Reading Greek

Athenaze : there are 2 versions, American and Italian. The Italian is far superior, has much more text and uses natural method more. I probably wouldn't recommend the American one.

Mastronarde

Hansen & Quinn (intensive course; intended for classroom use, I think)

You can learn with (nearly) any textbook, what matters is that you keep at it.