When you want someone to tell you their opinion, you say that you want to "get" their opinion:
Hey, can I get your opinion on this? Which one's better - the orange or the green?
what do you think?
You probably already know this, but you should ask "What do you think?" instead of "how". This can be confusing, because you use "how" in other situations:
How do you feel about that?
But just remember: "what" and "think" go together.