I got invited to speak at a conference, but I'm not sure I want to do it.
What were you invited to speak about?
About online marketing.  I have never spoken in front of a big crowd before and I am very nervous about it.
It's hard to condense a pep talk into nice little one liners. But I think if you just go for it, breathe deeply a bunch, and pretend it's just you in that room, I think you'll do great!