What's the first step in finding an adopted sibling?

Factual Questions
1

My Father had two sons before he married my Mother and put them both up for adoption. I only know their first names, the years they were born in, and that they were born in California and adopted together.

What are the steps I take to try to find them? Where do I start?

2

Finding out their last names, for starters. Squeeze your father for all the information he can provide. Maybe also see if there are Private investigators that specialize in this sorta things (yellow pages?)