Allow for a at least a year of learning how to maintain a saltwater environment. Go slowly. The hobby is not a science and the science is just trying to look at the iceberg. You need to come to a good understanding of the environment first then build on that knowledge. We have several members that have successfully started with cephs but they spent a lot of time learning before finally adding an octopus.
Since you think you want an octopus, keep in mind the tank size. Starting with a nano is a good idea but it will not support most cephs (there is one exception but the animal is not exciting or very interactive but
some people enjoy keeping this shy little octopus) . If you have an interest in corals, the nano can give you a place to put interesting things that should not be placed in a ceph tank. It can also become a tank for feeder animals or a number of other interesting things, including a
refugium or
sump for the future ceph tank. For an octopus tank plan on something of at least 55 gallons and preferably with a 20+ gallon sump (look up and learn the uses and configurations for the italisized words).
You will need to understand the
nitrogen cycle and the mechanical and biologic combinations we use to emulate an environment for ocean animals in a confined area. You will need to lean more than the simple chemical concept to understand how it applies to the aquiarium and how to recognize avoid mini-cycles through tank maintance and observation. Successful marine aquariums require consistent maintenance. Accepting that you will need to dedicate part of one day a week to the caring for your tank (or participating in your hobby, depending upon how you look at it) is something that is hard to learn if you have never had a demanding hobby. Although we throw a lot of mony into these glass boxes, there are few alternatives to the time required to keep them going.
You will need to lean to keep the water environment stable and consistent with the addition of freshwater (usually a daily necessity) and water changes to take out some of the pollution and refresh with new saltwater (usually a weekly or bi-weekly requirement). Additionally, you will need to think about how to obtain the purified water for both freshwater top-offs and saltwater maintenance.
I DO encourage you to read the tank forum for special requirements for an octopus tank but much of it will be difficult to understand until you have some of the saltwater basics under your belt.
There are tho very old achronyms and an expression I often quote about the hobby that you will find relevant as you learn.
NTS or New Tank Syndrome describes the effect of thinking you have a cycled tank because you see no Ammonia or Nitrites (and often told by your aquarium store that you can begin stocking). Most everything put into the tank dies within a month.
Nothing GOOD happens quickly in a saltwater environment.
MTS or MultiTank Syndrome occurs when you finally learn that not all animals can be kept together successfully. The affliction is common and many of us have it (myself included).