Assuming no other window manager / desktop envrionment is running: if you just start "i3" from the TTY, it will tell you that it cannot open the display because the X server is not yet running.
The easiest way to get started is to edit (or create, if misisng) ~/.xinitrc. If it didn't exist, simply put exec i3 in there. If it exists, check the bottom for some exec call, comment it out and put exec i3 instead.
Once the file is saved, run startx from the TTY. This will start an X server, read the file and then start i3.
Are you running some other window manager / desktop environment currently?