Thread adapter for Nikon D40?

I bought a Manto 190XDB Tripod for my Nikon D40. I opened it excitedly, only to find out my camera wouldn't fit on the screw part. My friend informed me I need an adapter. Unfortunately, I don't know what adapter to get- I don't even know what size I need. Does any one know what size I need to get my D40 to fit on the tripod securely, or better yet, the actual adapter (I'm on a low budget)? Thanks.