Questions looking for help with brand or model identification of a cycle. You MUST include a photograph of the complete bike. The serial number on its own is not enough to identify a bike.
Questions in this category should have at least one clear and well-lit photo of the right-side of the bike. If possible clean the bike before taking photos.
Also supply a clear photo of any interesting or odd features on the bike. Common examples would be the headtube badge, lugs, decals, seat tube/seat stay area, bottom bracket, and anything else that is distinctive.
Its possible the serial number can show useful info, but that totally depends on identifying the maker first. The serial number on its own isn't enough to identify a bike.
Before buying a bicycle of unknown provenance, you should always pass its serial number through BikeIndex or other stolen bike registry, as well as check with local police.
If you're asking about a BMX logo, then also visit http://bmxmuseum.com/ and look through all the relevant photos. A logo that looks like a B means the brandname probably starts with a B.