While I wouldn’t necessarily recommend using the mouse too often when using Tmux, it’s likely that you may want it to be enabled at some point, so it may be better to enable it before you need it.
The process is simple, you simply need a config file .tmux.conf in your home directory which contains the text set -g mouse on.
You can do this automatically with the following command:
echo "set -g mouse on" > ~/.tmux.conf
Congratulations! You now have mouse support enabled in tmux. You can now use the mouse to move between and resize windows.
Interested in not using the mouse? Check out the man page for tmux here (or type man tmux in your terminal).