What can I start doing now to ensure I will get a good job when I graduate?

Very good question. A straight and simple yet the meaningful one. At this age first try and figure out the interests of your life. Figure out what exactly you would like to see out of yourself 10 years from now.
This is exactly the right time to frame your thoughts. If you want to be a doctor start preparing and making your mindset accordingly right from now and the same applies with other career choices as well.
Good luck!.