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.