Welcome to MKO. There's a few things you should know and the first is that you should read the Community FAQ.
To introduce yourself, you should have posted a thread here (Introduction Discussion forum). And in regards to your questions, you should have posted those within the online section for the console you're using. If you check the forum lstings you'll see them as the PS2 and XBox online Tournament forums.
MKO is not a gaming website in the sense that you'll be able to play Mortal Kombat game shere versus anyone else. This is a forum and therefore you'll be doing as you already have done and making threads and posting within them.
To answer your questions, the yellow stars indicate the connection quality between you and the other person. Not everyone uses a headset. You'll see if they do via a headset icon that is shown when you challenege them. It's crossed out if they don't have one. And to challenge, justhighlight the person you wish to play and follow the onscreen button commands on the PS2 or XBox menu.

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Welcome to MKO.The place where your dreams...and nightmares exist.