Learn to Knit: Jeny's Stretchy Cast-On Tutorial
If you want a super stretchy cast on for the edge of your work this is a good one to start with.
It's frequently used for the top of socks and I've also used it for the edge of my Spritz Stripes sweater.
See it in action:
Photo Tutorial
1. Create slip knot and place on needle.
2. With left thumb lift working yarn from back, twist, and sit backwards loop on needle placing index finger inside.
3. Use thumb to bring yarn from front to back over needle and use index finger to lift first loop over yarn just put on needle.
4. Pull yarn to tighten.
Take care to keep stitches close together as it will be difficult to tighten up the stitches otherwise.
Read more about knitting cast on methods.