I look nothing like anyone in my family and have completely different hobbies, talents, and intersts. I also have never seen my birth certificate.

Well schimmy if you don't like the idea that you may have been adopted then I hope you have not been adopted, now if they have been great to you then it shouldn't be such a problem. All I know is that it sounds like it but then again who knows, almost no two people are alike or enjoy most of the same things you know? So don't stress OK? :)

I was completely different from my family as a teen.

EVERY teen feels like they were adopted. Every last one of us at one time or another wonders how in the world we could be related to our parents.

It is highly unlikely that you were adopted and don't know it. Open adoptions have been far more prominent since 1990 or so, so you would at least KNOW you were adopted.

Talk to your parents about your feelings.

Your birth certificate doesn't mean a thing, by the way. When children are adopted, the names on the birth certificate change to the adoptive parents' names.

Sorry, I have four boys, all of them as different as the day is long, growing up one was into rap music, another county western and another one something else.
One hunts and has guns, the other hates guns and is extremely liberal for animal rights.

As for as looks, my one boy who is actually adopted looks more like me, than my birth children.

Also adopted kids all have birth certificates, most likely there was no need to see one, on mine, I keep them in a safety deposit box. You had to have one to be enrolled into school and will need one when you get a drivers license.