About octo bites - you can search on this topic and find a lot already posted. Bimacs can bite their owners, usually when the octos are young and exploring things. It's a tiny bite, more like an insect sting - but, people who are allergic to toxins should avoid this. No one on this website has reported any real harm from a bimac bite, although it is possible that you could get an infection from it.
As for the food question, you need to offer some live food. You can find live food in various ways: gather your own along the shore, if you're so fortunate as to live by the ocean, or order shimp or crabs online, or stock a feeder tank. It's important that you resolve this issue before buying your octopus.