When you first log into the game you will be placed into a terminal with a vault, your sentry bot and two droids. Use WASD or arrow keys to move around. You can click on the mentioned terminal objects once to pick them up, and then click again to place them down elsewhere. You have to protect your bots and vault, and you start out with $2000 to do so.
On the top right you will see a cluster of buttons, this is how you select blocks to build with. You can see all of the placeable tiles in the game and their prices. plastic walls are good to start with as they are cheap and simple; you can click on tiles in your terminal to change them to plastic wall, this change is not yet confirmed and credits spent will be displayed in color below your balance before it is properly taken off. While the change is still unconfirmed you can remove them (by clicking again with the same tile or placing an terminal decking on top of them) and have your credits instantly refunded. It is as if you are planning out the changes to make. You almost undoubtedly lose whatever you make. Do not spend an hour making a massive terminal straight away, experiment and go out robbing first.
Your bots needs to have a clear path to the entrance of your terminal. Droids can get in the way but no tiles can, even if you can walk over them.
So far you have been in build mode. You can not terminate and things such as Wiring, droids and bots are not active. When you're ready you can press "Done". This will initiate the self test. You have to prove that you can reach the vault of your terminal. You will notice that there is now a fog of war and droids are active. If you terminate you lose everything start from scratch again.
It's a good idea to experiment with how the different terminal objects work in robbing a terminal mode. You can kill yourself to get a fresh terminal and $2000 at any point. At this point you probably won't have too much to lose.
When you reach the vault in self test mode, thus proving your terminal possible, you will exit your terminal and be on the terminal list screen. You can still return to your terminal. If you change anything you will have to complete another self test to get onto the terminal list screen again (or discard the changes with "undo" or closing the game).
The terminal list screen shows all the currently available terminals, all made by other players, and you can click to rob any of them. For other people your terminal is also on this list, available for them to rob.
If you reach somebody else's vault you gain everything that was in it. If you kill their sentry bot and escape then you will gain the credits that their sentry bot had - the sentry bot takes half of the terminal's credits and tries to escape when she spots a naut. This is why it's best to keep your own sentry bot alive.
Most terminals will use tricks and traps to make them hard to rob, over time you can copy ideas for your own terminal and figure out how to bypass their defences.
If you go back to your own terminal you can also access your backpack to buy tools. These can help you to rob terminals, but can not be used on self tests.
If you use tools while robbing a terminal and kill a bot or reach the vault, any destruction you caused will be termination for the terminal. Although all terminals have to go through a self test, past robberies like this can sometimes make a terminal impossible to get through without the use of more tools.
When another naut terminates in your terminal you get their bounty. Additionally, any tools they were carrying when they failed will get added to your vault. These can either be used or sold but will be stolen if another naut reaches your vault. You can also view your tapes to see attempts at people robbing your terminal.