You should get your setup, get familiar with keeping your tank and water in good condition, and then focus on your octopus. It's ALL tricky at first, and it will be really hard to try and juggle all of those chores at once.
It sounds like you have a "chain" store, like a Petco. I'd try to find a store that specializes in aquariums and aquatic creatures. Those employees will almost definitely be more educated on the subjects you ask questions about. Like Jean suggested, maybe you can find a city aquarium close enough you can visit and talk to the staff, maybe even volunteer. That would be the most ideal, but I know those type of places aren't real convenient for everyone, unfortunately.
Click on the "Articles" link underneath the large TONMO logo at the top left of the screen. There are some great articles there to get you started.
You're probably going to want to go with a captive bred octopus if you want the longest lived specimen. There are members here who can supply you with one when the time is right. Keep in mind, you may have to wait weeks, or even months before you can get one. Otherwise, you have to order one from an online store, and you won't know what you're going to get or how old it will be. This is going to be the case most of the time, the only down side to keeping cephs.
Reefcentral.com has some really good information! You might do some reading there too. I studied up and started keeping saltwater tanks about 2 years before I got an octopus, and still, there's a lot to be learned with the actual experience. Be careful what and where you read, because there is a lot of misinformation on the web. If you can find a copy of the June 2007 issue of Tropical Fish Hobbyist, there are several really great articles in there also.