I like the yarns spun especially for socks with 75% wool and 25% nylon (for longer wear). Even in the summer, wool is a good fiber for socks, as it wicks away moisture.
Sock yarns come in solids, prints, and stripes. Some yarns are put up in skeins that will make a pair of socks; others will require two skeins for a pair for an adult. If you decide on a self-striping yarn or other distinctive pattern, be sure you decide ahead of time if it's okay with you if your socks are fraternal twins instead of identical twins. For me, they have to be identical, and it takes extra time and effort to match the starting points for both socks.
Brands of sock yarns I have used and liked include Opal, Plymouth Sockin' Sox, Regia Design Line, and Red Heart's Heart and Sole. I have some Patons Kroy that is "in line" for the next round of sock knitting. I have also knit a number of socks from Plymouth Encore, a worsted-weight mostly acrylic yarn.
The sock yarns usually come in a 50-gram ball at around 225 yards that makes one sock or in a 100-gram ball at around 250 yards that makes a pair of socks. Since I knit both socks at the same time, I rewind half the yarn on the 100-gram balls into a second 50-gram ball.
Where to get patterns:
My tried-and-true resource for sock knitting is the sock pages in Ann Budd's The Knitter's Handy Book of Patterns.
You can get a more complex Ann Budd pattern free along with patterns from Lisa Shroyer and Priscilla Gibson-Roberts at Knitting Daily (http://www.knittingdaily.com/blogs/patterns/archive/2008/12/17/5-favorite-sock-knitting-patterns.aspx ). You do have to register for Knitting Daily, and it's a benign site. While there, you might scroll through their free patterns for the other sock patterns: http://www.knittingdaily.com/blogs/patterns/default.aspx
Plymouth Yarns has easy free patterns for slipper socks and children's socks at http://www.plymouthyarn.com/index.php?nav=cPatterns.freePatterns&pattern_id=000027.
Knitty.com has everything from the whacked-out (Frankensocks and Space Invaders) to the elegant (Baudelaire), top-down or toe-up, at http://www.knitty.com/archiveFEET.php
And the yarn company you choose probably has a free pattern for its yarn as well.