Considering that's the one thing I battle between constantly with, medicine or physics after UG I'll chime into the best of my ability and hopefully it will be of use.
I don't know exactly how the Philippines does it, but I imagine it to be quite similar. You take what is normally 4 years of UG in any topic of your choosing. Some schools allow early admittance (i.e. 2 or 3 years) but if you do get in early, you need to be phenomenal in every aspect. This is where I'm unsure of the differences. In Canada, your UG does not matter, no UG is looked at as "better" GPA trumps all. In the US your UG is taken into consideration. If you apply to medicine with a 3.7 in physics, it looks way better than a 4.0 in biology. So you will want to research into that aspect of it. You'll also want to see if schools there have required courses. Many Canadian schools removed required courses to apply, while some still have them and most of the US schools do as well. So see if your schools of interest have any that you need to take so they can be put into your schedule somewhere.
You need to take your MCAT at some point before your 4th year, so no later than your 3rd summer. They've just recently changed what it consists of, so take a look at the topics and go from there. Courses that are normally recommended that you take before taking the MCAT are: Biology I and II. General chemistry I and II. Physics I and II. Organic chemistry I and II. Biochemistry I. Psychology I, and sociology I. While they aren't required to do well on the MCAT, they can help so you aren't doing so much self studying. If you're going into physics it might not be that easy to take these classes.
Volunteering is a huge chunk of the battle. While GPA is king, even if you have a 4.0 and no volunteering, or research or any EC's you won't get in. So, make sure that you are volunteering with professors, or outside of the school in the community. It doesn't matter what you do, but, it does matter that what you do is something you really enjoy doing. When they ask you about what you've learned, if you've done it so it looks good on your CV not because you loved it, it'll show.
It's most certainly a marathon, so take off your jet pack and just keep plugging away, you're in for a long run.