To simply believe in him. John 3:18 seems to say you must believe in his name though, but since Romans 10:9 says if you confess with your mouth "Jesus is Lord" and believe in your heart that God raised him from the dead, you shall be saved I suppose his name might include that he is Lord and that God raised him from the dead (this is not clear from John afaik so I don't think this is what his name is) or that you must only believe in part of his name, or through his name, ie. believe in his name. This seems to be what things are saying, it also seems to be evidenced by that you must "believe on him" (John 3:18 KJV). Sorry if this was confusing.