As others have mentioned in this thread, you should wait until you've taken some courses before you start seriously worrying about where you're going to end up after school. You might be interested in something when you start, but soon realize that it's not what you thought it was going to be. This is normal and happens to a lot of people. I started out thinking I was going to be an EE, then went Engineering Physics, and I'm now settled and quite happy in math and CS. As you take more courses, you will find out what truly interests you.
I'm of the opinion that most people go about their education backwards; they choose their career and then take courses. Many of the posts I read are by people who think they already know for sure what they want to be (which is very difficult since most of them haven't even taken a course that is specific to their major, let alone finished the standard Calculus sequence). So they start taking courses and find out that the problems their specialization attempts to solve are not as interesting as they thought they'd be, for whatever reason.
I believe people should declare something that sounds interesting to them, but not write it in stone; let your courses decide your future and
be open to changing your mind. You might find out that you have more of an affection for logic and proving theorems than you thought, as was true for me.
Sorry for the lengthly anecdote, but I hope it helps justify why I think you should slow down a bit and wait until you've taken some courses before you start worrying about a career.

Declare Mechanical Engineering or Computational Mathematics for now and see where it takes you; at this point, it is not a life-or-death decision and you have time to alter your path should you find it unappealing at some point.