WordPress default editor have no support for any font change option on it’s tool bar. You have to do some css tricks but its not so easy if you are not a developer. Also its not a good idea to change theme functions or css.
But there have a good solution:
You can do this easily with a little but powerful plugin like This
You can also check an intro video from Here
Hope it will solve your problem.
Note: If you don’t want to use plugin like this then you have to change core css of your theme which is not a good practice.