if you start it up you'll see some text at the top:
- Username
- Server
- Bindings
next to all of them you'll see the cute little "->" button which is like "Config". click it to change the settings.
1. Username
Set this one as you want your nickname, obviously. but in there you can also change your phonetic, which is what the computer will say when you join a server. ex, "X joined the channel" etc. the rest is kinda useless unless you want to get into it.
2. Server
Here's the important stuff. You'll have a scrollist up at the top to switch between them, press "New" to add a new, obviously. just set the ip and port to what bronx just mentioned

the name you pick for it is not necessary, it's just for you to know what server it is in your list. it's probably easiest for you if you name it "13th MEU" or something

you don't need to care about password since you're a guest, this is just something that'll give you access to member-privilages if you get your own password later.
3. Bindings
this something you never really need to ever check unless you're really curious, in my opinon there's nothing useful there.
Something that you want to do before you join is to check in the menu on the right, go into "Setup". under the voice tab you setup your mic and soundsettings as you want them. in my opinion a push-to-talk key is vital so nobody hear you snore if you fall asleep or whatever with voiceactivation

also, if you activate Direct Sound by pressing the boxes and fixing your soundsettings you can later also rightclick on a user and pick "Misc" -> "Special effects" and add a volume, this way you can adjust a specific user's volume in case he's stupid enough to not do it on his own in Setup

but yeah.. I think that cover the basic usage? O_o there's a lot you can do in the setup, so if you want to go ahead and check it out. but with all this you can use without problems really, and it's always nice to use the test-function under the voice-tab in Setup to see if your mic sounds alright before jumping into a server =3