I am new to i3wm, so would like to know which command to use to access the files on my system.
Well it depends on which file manager is there on your system. All answers assumed it to be "nautilius" without the user explicitly specifying his system specifications (as different flavours of ubuntu comes with different file managers like pcmanfm in LUbuntu etc).
So I would suggest you first get the knowledge of the file manager on your system and proceed as follows :-
Just go to the terminal by pressing Mod + Enter key combination (this will open up a new terminal) and type the command for your respective file manager program.
or you could just simply open "dmenu" by pressing Mod + D and enter the command there.