I'm writing a Salutatorian speech, and would like to say something to the effect "I'm up here and I don't know why." However, I don't know how to say it without offending anybody or seeming rude... help?
I don't see anything wrong with, “I'm up here and I don't know why.” It sounds natural; it would catch my attention. (I imagine the next part of the speech would explain why, to some extent or another.)
Just my two cents: Strive to speak from your heart, rather than seeking words of eloquence. The speech will be easier to deliver, and more enjoyable to hear.
Perhaps, though, you mean to say:
I'm up here – and I'm humbled to be here.
I'm up here – and it's hard to believe it.
I think you should try to be natural and write/say what comes to you naturally. Though you might want to come up with a great speech, those hearing it may prefer to get a few laughs out of you and I think your original 'I'm up here and I don't know why' phrase might just do that.
However, if you wish to be more poetic, consider this, 'I can't believe that I'm standing up here, it's such an honour,' or something to that effect.