Pretty self explanatory
How can you use a Mic?
1) buy mic from store
2) Plug the mic into your sound card. You will usually plug the jack into the pink port on the back of your computer. You might have to install additional software to get the microphone working, but not always.
Now you should be ready to go. To transmit your voice over the internet, you'll need some type of VOIP software. Ventrilo and Skype are popular choices. I heard you can use steam for that purpose, but I don't know how to do it.
I use the Steam client and it uses the USB mic I have as well; no external VOIP software necessary. Besides, in order for those to work, everyone else would need to be on it as well; this all works in-game and on the fly.
Again, I'm on Steam, so can't speak about non-Steam version.
As far as I know, only Steam users have voice chat support. OverDingle pretty much summed up everything, but if you do not have Steam, you can not use voice chat.
I would use an older version of Roger Wilco... file version 1.4.1.6 Google it and you'll see it from file planet. (or lots of other places) It's a great program ... I've used it with other games like Crimson Skies and Unreal Tournament It allows an entire Guild to communicate hands free if so desired.
If you have a microphone, sure? I don't know how or if that works for non-Steam users.
As far as I know, SK borrows your steam mic settings, so you need to make sure you're using the steam version and have working mic settings for steam - make sure you set up your mic properly in steam's settings.
From there you just turn it on in the options in-game.
That said, I've never used SK outside of the steam version, but the in-game option for mic is under the steam tab in the steam version, so if there is a mic option in the non-steam SK client, it works differently and I can't help you with it due to lack of experience.