What's your default EDITOR (echo $EDITOR). If it's not vi try:
export EDITOR=vi
and then try crontab -e again. Incidentally, crontab -e isn't the safest way to edit your crontab if you're new to the game. You could:
crontab -l > newcron
then vi (edit) newcron and apply it using:
crontab newcron
Good idea to keep a copy of the original crontab -l too, in case anything goes wrong.