When something surprises you, you say that you "were surprised by" it. "Surprised" describes your reaction. If you're describing the situation, you say "surprising":
It was surprising how easy it was.
I was surprised by how easy it was.
There are a lot of words to describe emotions that work similarly to "surprising" / "surprised". A few examples:
- It was embarrassing. / I was embarrassed.
- It was interesting. / I was interested.
- It was amazing. / I was amazed.
Each word comes with its own preposition:
- embarrassed by
- interested in
- worried about
"Surprised" uses the preposition "by". The best way to learn which preposition to use with each word is to memorize examples of each one.