Terminal: Where is the shell start-up file?

You’re probably using bash so just add these 3 lines to ~/.bash_profile:

$ cat >> ~/.bash_profile
export WORKON_HOME=$HOME/.virtualenvs
export PROJECT_HOME=$HOME/directory-you-do-development-in
source /usr/local/bin/virtualenvwrapper.sh
^D

where ^D means you type Control+D (EOF).

Then either close your terminal window and open a new one, or you can “reload” your .bash_profile like this:

$ source ~/.bash_profile