If this would be your first SW setup, I would not do it. Octo's are tough critters, but they need quality water. And, since they have short lifespans, it's a lot of work for a cirtter that's going to die off in a year or two.
Having said that, basic setup is pretty simple. At least a 30 gallon tank, even for a bimac. Crushed coral or sand--crushed coral is easier for your to clean up with a siphon. Live rock--2 lbs per gallon for primary filtration, otherwise just a few peices so the octo can build a cave and hide out. Definitely have a canister filter setup for emergency use, if not running for biological filtration. If the octo inks, running carbon is your best bet to remove the ink quickly.
Minimal lighting, powerheads for some circulation, but make sure the intakes are covered and secured. Openings should be covered, and do NOT keep food sources in sight of the tank. Heaters are better kept outside of the tank in a sump, IMO, but otherwise, make sure the temp adjustment dial is fixed or outside of the tank--don't want it turning the temp up on itself.
Do not feed goldfish--they are one of the few SW animals that can metabolize the fats, but goldies still are not a great source of food. Ours would all take ghost shrimp with relish, hermits happily, and thawed frozen fish fillets as well.
If possible, order one from a breeder, and get a male. They last longer, and don't deplete wild populations.
