Question

how did muhammad first begin to spread islam

Answer 1

http://en.wikipedia.org/wiki/Spread_of_Islam
Islam Begin with Muhammad Profecies and it spreading, in VII, in Arabia.
It spread through militar conquest, made taking advantage of the weakness of the two big Empires of that age, Byzantine and Persian-Sasanides. Also Islam has a peculiar law : while as Christian, or Buddhist or Indueist, you can convert to other religions, Muslim are automatically condamned to Death penalty if doing so, even nowadays in country following Sharia (Islamic law). An effective way for spreading, isn't ?