:w save (write to disk)
:q exit
Make sure you are in command mode by pressing the 
key. Now
type :w. This will save your work by writing it to a disk file.
The command for quitting vi is 
. If you wish to combine saving
and quitting, just type :wq. There is also a convenient abbreviation
for :wq -- ZZ. Since much of your programming work will consist of
running a program, encountering a problem, calling up the program in
the editor to make a small change, and then exiting from the editor to
run the program again, ZZ will be a command you use often.
(Actually, ZZ is not an exact synonym for :wq -- if you have not
made any changes
to the file you are editing since the last save, ZZ will just exit
>from the editor whereas :wq will (redundantly) save before exiting.)
If you have hopelessly messed things up and just want to start
all over again, you can type :q! (remember to press the
key first). If you omit the !, vi will not allow you to quit
without saving.