Who do you plan to talk to?  Find out what they use.
The 2 biggest are MSN Messenger and Yahoo Messenger
Download the program install it and run it, it will then guid you through creating and account.  It's all free.

Y! IM or Yahoo Messenger is my personal favorite. I've had an account with Yahoo in some form now for roughly a decade and I just love their service! 

http://messenger.yahoo.com/download/

Click the link that says DOWNLOAD NOW it will be the newest relase 9.0.0.???? During the installation process you will be asked if you want to do the typical or a custom installation. Fear not it's not hard so click the blue hyperlink that says custom installation and UNCHECK every box EXCEPT create DESKTOP SHORTCUT. You don't want Yahoo installing toolbars and shortcuts all over your system. They're not needed and can clutter up even the best system. :)

Once the installation is finished you will be asked if you want to run Y! IM. If you don't already have a Yahoo account:

https://login.yahoo.com/config/mail?.intl=us

3/4 the way down the page on the right hand side there is a SIGN UP link. :)

Best of luck!

Sean