This tutorial should make your start with octopus a little easier. The tutorial will provide you with a couple of examples of different things that you can do with this program. It is by no means complete, there are a lot of other things that octopus can do for you, but hopefully it gives you an idea of how to use the different options. You can find more information in the online manual.
octopus is a real-space code that works with space grids. The spacing can be different in different directions. All quantities are given in atomic units unless stated otherwise. It also treats all numbers internally as double precision but you as the user don't have to worry about that.