I normally use vim. I recommend it over vi; it really is "vi improved." I'm hardly expert, but I'm competent enough that vim is my now my preferred editor.
ESC-i will allow you to enter text one character before the current cursor position. (I have gotten into the habit of hitting ESC before I issue a command or enter text; some tutorials recommend that you hit ESC when you have completed your current operation. It is, methinks, whatever works for you, so long as you remember to ESC.)
This link should help:
http://www.radford.edu/~mhtay/CPSC12...r_Commands.htm