You can use this in your shell dotfile (e.g. ~/.bash_profile, ~/.zshrc) to create a screen session when you log in or reattach to an existing screen:

if [[ $TERM != 'screen' ]] ; then
    if [[ `screen -list | grep -v "No" | awk '$2 { print }' | wc -l` == 0 ]] ; then
        screen
    else
        screen -dr
    fi
fi

This works in bash and zsh.