I'm really sorry. I'm not going to write a speech for you because I don't know the first thing about your brother, but try not to talk about "god" or anything that could be controversially mistaken - I had such situations. Say only good things about him etc. You could also talk about what he told you he wanted to be too. That's common on those kind of speeches. Try restraining yourself from crying during the speech, as it's likely most people will follow. Good luck.
EDIT: You could also tell a light joke to lighten the mood and credit it to him.