I am guessing here, you are a Canon user. As far as I am aware all other manufacturers include a lens hood when you buy the lens. So as KeeFy says, go look at the site and it should give you the details of which lens hood you need. Most good camera stores will also have a reference chart, so you can always go into one and ask them.
"It is one thing to make a picture of what a person looks like, it is another thing to make a portrait of who they are" - Paul Caponigro
Constructive Critique of my photographs is always appreciated
Nikon, etc!
RICK
My Photography
For Canon lenses, this is a good resource for lens info including recommended hoods.
http://www.usa.canon.com/app/pdf/lens/EFLensChart.pdf