Here is few lines you need to add to your .bashrc/.zshrc/.whatevershellrc:

if [[ -z "$TMUX" ]] && [ "$SSH_CONNECTION" != "" ]; then
    tmux attach-session -t mysession || tmux new-session -s mysession
fi

These lines check if tmux session mysession is exist then we tmux attach-session is executed. Otherwise new session with mysession name is created.

Replace mysession to something more meaningful.