After you have installed mumble and launched it, you will be guided through the configurations, just follow the instructions and you are almost ready to go.
There is just a few things in the settings you may want to change:
Go to configurations and set the compression quality to about 40.0 kb/s and audio per packet also to around 40ms, this has been found to be a good setting. Higher values in compression give you better quality, but there is almost no hearable difference above 40.0 kb/s, you may can go up to 60.0 kb/s if you really think you need better quality. Audio per packet means, that he lower the setting, the more packets are send, so the lower this setting, the more overhead in bandwidth you get.
Push to talk:
A good practice is, to set mumble to push-to-talk mode, this means you press a button and only then your voice is sent. You have to go to keyboard shortcuts and create a new one, chose "push-to-talk" and add your desired key, I would recommend something that is not needed in other programs, for example the right ctrl-key or a button on the mouse, if you have more than the regular two on it.
Under Messages you can switch sound notifications on or off, this may be useful. Mumble has also a text to speech function, that will read messages for you in the chat, it is funny, but you likely want to turn it off, this can also be done under this section.
When you are finally on a server, you also can change your avatar or your comment/profile, this can be done on the second menu entry or on othe big yellow "C"-button.
Basically that's it, those were the most important features you need to know,