I'll actually second what FE said and recommend that you not return here for a year or two. It'll make you paranoid and, frankly, your first semester or two should be focused on making the adjustment to college successfully and doing well in your courses rather than getting immediately to the business of building your resume.
That said, I think these considerations should always be on your mind as you go through college:
1) What am I doing to build my resume?
2) How can I get clinical experience?
3) How can I get leadership experience?
4) How can I meaningfully get involved in things that I care about?
5) What can I do to make good use of my summer time?
6) Am I getting too focused in one particular area?
For #1, do well in your classes and, most importantly, do things - no matter what they might be. This is critical yet can be hard if you're somewhat apathetic (like me). Personally I have no problem hanging out all day with friends and doing absolutely nothing that I can point to and say, "look at what I did!" If this is you, you MUST force yourself to do things that you can talk about on an application. No, you don't always need to be doing these things - I spent (and still spend) plenty of wasted time on video games - but you need to make sure that wasted time is balanced with productive time.
For #2 and #3, you shouldn't be immediately concerned with getting these things knocked out. In other words, if you haven't accumulated clinical experience during your first year, by no means are you screwed. Again, the key is to do these things at SOME point and for a length of time that shows that it wasn't simply something you did to check it off the list (...even though it inherently is).
See above for #4. Do things you're interested in. Be involved. Get leadership positions. Be liked by your peers and superiors. This will lead to experiences that you can talk about meaningfully on your application and interviews. Most importantly, these DO NOT have to be your "typical" ECs. In fact, if your goal is to get into a top-notch school, I would almost say that doing the "typical" ECs will be hurtful. The ultimate point, though, is to do things that demonstrate in a tangible way that you have interests, that you can work well with people, that you galvanize a group of people around some kind of cause (whatever that may be), and that lead to some insight into your general character.
For #5, just do SOMETHING during your summers. It doesn't have to be in a lab or at a hospital. See the above paragraph; it just needs to be something you genuinely care about and is a productive use of your time. Maybe it's doing some kind of community service project... maybe it's some kind of hobby/EC that you're passionate about... who knows. Just don't spend the majority of your summer time sitting on the couch watching TV. Summers are excellent times to get things done.
For #6, this is the oft-repeated "well-rounded" characteristic schools are looking for. Try and do a variety of things. The main exception to this is if you plan on applying to MD/PhD programs, in which case your application will almost certainly be unbalanced as you'll more heavily engage in research.
I know this is all very abstract, but there isn't necessarily a checklist of things that you need to do (...and yet there is). If I had to give you three pieces of advice, it'd be these:
-Do your absolute best in your classes. This may prevent you from partying like your less motivated peers, but that's the way of the world.
-Do activities that you genuinely care about and yet are "serious" (for lack of a better word) enough to be included on an application to professional school.
-DO SOMETHING DURING THE SUMMERS.
Hope that helps. Relax.
