Einar Rognstad (Amundson?)

We are trying to do my husbands family tree and we haven't got any idea of how to start. We think that there were name changes and we also don't know when they left Norway. His grandfather's name was Einar Rognstad (or possibly Amundson) and he came to America as a young child. He traveled with his mother only. Where do you suggest I start? Any ideas would be wonderful!!! Thanks- Rognstad Lynn Rognstad