Have you started the form yet? Most of the questions should be pretty self explanatory. Generally should fill out the form for the country you are a citizen of (or where your company is incorporated if applicable). It is not necessary to apply for a US tax number, but, depending on the country you are from, you may be eligible for a reduced withholding rate. I don't think Bangladesh is eligible for this though, but to confirm you should do some research online and/or talk to a professional.
Hope that helps