Hey Kate,
Because there are so many variables in design work, the best advice I can give you is to try to talk to as many people as you can in each area of each field.
Your going to find that even if you decide on one design field, there are going to be a lot of different work industries that you'll have to choose from. For example, with graphic design: marketing/ad agency, freelance, publication, POP (point of purchase, such as product packaging design), web.
I took the first job I applied for at a small printing company. The pay was below the avg for a graphic designer, but I really learned a lot about how my design work translated to the printing end of it - having files prepared correctly, sizing issues... It was a great learning environment, and it was cool to see my work go from the computer to the finished product (plus a great way to get samples of my work). When I was laid off, I decided I wanted to try something a little more "chic" and applied at some marketing/ad agencies. I found that despite the experience and portfolio, those are hard nitches to get into, unless you are young (and moldable), or from a similar background. I now work for an association and make good money. Associatoins are a nitch that I had never even thought to explore (heard about it through a friend), and wished I had done more researching at the beginning of my career to find the "hidden" treasures.
So my advice is to find a couple of business close to you and ask to job shadow or for an informative interview. Ask questions like: what kind of education do I need, what kind of programs/equipment do you use, are there slow periods in work, whats the best part of working in this field, what are the different levels of advancement, what makes your company stand out from the competition.... and always end with asking if they can recommend anyone else to speak with (another company or someone in the same line of work, or even an organization or local club).
Once your in college, ALWAYS bring a portfolio and business card. It is ok to ask for your interviewer or person your shadowing to look at your portfolio and offer advice. Plus it gives them a chance to see what you do, and maybe they'll give you a call if they get an opening they think your qualified for.
Let me know if you have any other questions (BTW, Macs rule! but PC's will do if thats all thats available <img src="i/expressions/face-icon-small-smile.gif" border="0">