Where to begin to find Sigvart Halvorsen?

My husband and I thought it would be very interesting to try to find out about his Grandfather. However, we know VERY little about his grandfather. His name was Sigvart Halvorsen. My father-in-law believes he was born around Bergen. According to the Social Security Death Index, he was born November 1, 1898. But, I've found inconsistencies on that index. He came from Norway as a stowaway to New Orleans, LA. He had two brothers, at least. He never spoke of his family and spoke very little English. Can anyone help me start? We have no documents or anything. I would really appreciate any pointers or advice. I know it's very little to go on. Thanks