The usage of "till" and "until"

She was not going to arrive until noon.
'til is informal.

Both till (double 'l') and until are acceptable (and largely interchangable).

'til is a contracted form of until and should be avoided in formal writing.

So how can I say my own sentence?

I think that AlpheccaStars has it right, but you haven't provided context.

She was not going to arrive until/till noon.

I wanted to say she was going to arrive till noon, but she hasn't arrived yet. (now it's afternoon)

I wanted to say she was going to arrive at noon, but she hasn't arrived yet.
Until is not a suitable preposition.

Looking to the future:
Our guide will be with us until 3 pm.
We will stay in the hotel until Thursday.