You need 3 string and 3 sticks. You put the strings in a vertical line on the lefthand side and the sticks in a arc shape on the right. It should look like this:
[String] [Stick] [empty]
[String][emtpy][Stick]
[String][Stick][empty]
Also, check out more details and a picture representation of the recipe here: http://www.minecraftwiki.net/wiki/Bow