"Mail" is the default email client for OS X on any MAC computer. I actually quite like it - it's much nicer than Microsoft Outlook in many ways, including the way it shows a little red notification on the dash to show when new email has arrived. Here I've entered some tips on how to setup Mail and also import contacts.
Setting up a New Mail Account
Rather than type it out here, you can find good instructions on this page: Setting Up Mail for Mac OS X Using IMAP. These instructions are specific for UCSD, but the should help for any account.
Importing Addresses in the Address Book
Mail uses the Address Book Application to help autocomplete whenever you send someone an e-mail. To import into the address book is a little tricky - it doesn't really support CSV (comma separated value) file formats - only its own "vCard" format.
If you're using Yahoo mail (like me) you can import *most* of these addresses into Apple's Address Book by going to Contacts > Tools > Export the use the "vCard, Zip of .VCF files" option which generates out ALL your contacts as "first_last.vcf" under one zip file. After you unzip this you can then use select all and drag them all into your Address Book. What I've found with this process, however, is that not not all of my contacts imported and it tended to add junk at the end of those that do. To solve this I exported to a gmail account using a CVS file.... and then exported this to Mail.
If you're using Gmail simply go to: Contacts > More Options > Export then select "vCard format (for importing into Apple Address Book)". This works perfectly as Apple Address Book opens it automatically (without you needing to do anything) and ALL you contacts will copy across seamlessly (albeit without any pictures).
As far as I know there's no way to import contacts without losing any contact pictures you have attached, but I'm hoping to find out a way a some stage.