Now that you have your yarn and your needles it's time to learn how to cast on. There are many different ways to cast on but as a beginner you should try the backwards loop cast on. It is simple, easy to learn and it gets the job done.
First you will want to place a slip knot on your needle. If you do not know how to make a slip knot check out the tutorial on Wiki How.
Here is how it should look once your slip knot is on your needle:
Next you want to take the live end of your yarn (the length of yarn that leads back to your ball of yarn) and wrap it around your index finger. As you can see in the picture below my live end is under the length of yarn coming off the needle.
Slip this needle up through the loop from underneath.
Now the easy part. Slip your index finger out of the loop and pull the loop tight around your needle.
Make sure you don't pull your loop too tight. You want the stitched to be snug around the needle, but you want to make sure you can still slide the stitches along the needle and get your second needle into the loop with out difficulty.
Tomorrow we will learn how to knit and a simple project pattern. Go practice the backwards loop cast on. Below are some more resources for this type of cast on.
More Backwards Loop Cast On
Learn To Knit: Backwards Loop Cast On, a great video on how to execute this type of cast on.
Wiki How: How to Make a Slip Knot
Knitting Daily: Backwards loop cast on, short, sweet and to the point.
Knit Wiki: Backwards loop cast on