2d8 is its racial hit die (2 HD specfically). As an animal its HD are always d8s.
The normal Pathfinder rules are pretty vague about using monsters as PCs, but they do provide some guidelines.
To answer the questions from the original post:
At 1st level you would probably be just an octopus, as it is a CR1 creature with racial hitdie. At 2nd level, you would take your first level in a class. Your attacks would come from the CR1 octopus statblock. Your stats would also come directly from the statblock with no adjustments to the ability scores (unless the DM decides that you should be allowed to roll ability scores or point buy, in which case the DM would determine appropriate racial modifiers in line with what other races recieve (ie. +2 CON/WIS -2 CHA)).
Most of the specifics of how this would work are up to your DM as playing as a creature with racial hitdie is largely unsupported ground in the normal rules.
I'd probably make it an octopus which has gained intelligence via the Awaken spell. If your group rolls stats maybe that's enough adjustment right there.
If you want to use point buy I'd say it seems fair to assume that the octopus has at least a +2 racial modifier to Dex. What sort of modifiers should apply to the Int and Cha of an octopus which hasn't been the target of Awaken is tough to say, maybe -8? If it is Awakened I'd probably just assume +0 for mental stats.
