Know that you're going to enjoy it only if you're passionate about most things involved: the science, the patients, your colleagues, a life of learning. Obviously, no one's passionate about bureaucratic bull****, but you do what you can.
Major in what you're interested in now. Don't major in biochem unless you're really into memorizing pathways. (Personal experience...I should have done p-chem, chem, or physics.) You'll have plenty of time to memorize tons of junk, might as well study something interesting now.
The hardest thing for me in my preclinical studies is remembering that I'll (hopefully) actually enjoy medicine and that this isn't really a reflection of my career. I really hope my career won't involve only memorizing. It's hard because I've been doubting if I'm really up to spend huge chunks of my life working because I'm bored now. I really do think I'll love what I do and be happy to be spending long weeks doing it. That, or ophtho.
If you're sure you want to do it, know that in the app. process you need good grades and scores and all that, but that you can distinguish yourself in lots of other ways, too. Not everyone needs to publish research papers or whatever. The only hard and fast req. for ECs is clinical experience. Beyond that, play in an orchestra, study abroad, do whatever floats your boat. The only real caveat to that is that you gotta be able to write it up well as an activity on your app.
