Bob, I use a cheap "LabTec" computer mic. It is available at Walmart. It's all you need for chat rooms and "messenger".
It is not "USB". It plugs into the mic jack on back of the computer.
Chat rooms aren't as critical, but for Skype or any other VoIP application, you should use a headset. Otherwise, you're likely to get echoes. Also, the folks on the other end of the call will notice a very hollow sound, similar to a speakerphone.
The worst sound quality combo for VoIP is a laptop with built-in speakers and mic. Slightly above that would be external speakers and a standard generic PC mic.
I'd strongly recommend you get a USB headset rather than one that plugs in to the sound card. Among other advantages, you won't lose use of your PC speaker(s). Logitech and Plantronics both make inexpensive USB headsets that are Skype certified and will offer vastly improved sound quality over separate speakers/mics.
The Skype store has several certified models for sale, as does Amazon.
I had a cheapo mike (Lebotone, or something like that) which connected to the "Audio In" jack. It worked okay, but the chat room's mike volume control wouldn't work with it. Wifey got me a better (Logitech) USB mike, and that works fine and sounds better too, according to listeners. A side benefit is a handy lighted "ON-OFF" switch on the stand. Cost about $30.