A 20 POUND shelf? I think that will need more than 2 holes/screws at the top to hold it up. Is the shelf already built? Take a photo, print it out on regular paper and take it with you:

Why not have a chat with some of the folks at a local FYI store (Home Depot, for me). I've found most of them are QUITE knowledgeable. Since I bought my house, I've learned which people are best to ask, as well.