Don't worry...Be happy. I would suggest learning a variety of computer languages, not only because astronomers increasingly need to program in C and C++, but because if you don't make it in astronomy, you can always find a 'real job' in the so-called and much vaunted REAL WORLD.
In the astronomy program, you will meet people a lot smarter than you. You will not meet too many dummies, so you need to make certain for survival sake that you are as close to the top as you can get. Don't use drugs or ANY kind of recreational ( HA!) substances. You don't see concert pianists out there chopping logs, learning judo, or taking uo amateur boxing! Your brain is your ONLY ticket to a stable professional secure job in astronomy. Don't screw it up by taking any chances with 'mind altering' substances. Your friends may say its OK...but hey...they are NOT going to be astronomers!!
As for studying, learn everything you can about what is happening in astronomy. Build up an intuition of what is hot and what is not, and where your interests lie. Hot research areas get rewarded with grants and money, and this is where you want to be. Save the 'big deep thoughts' for when you retire or get tenure. The race to get tenure or a secure position is EVERYTHING it is reported to be.
Get into the best graduate school you can. This doesn't have to be Ivy League. Remember, your grades also count for something. Doing mediocre work at Harvard does count less than B-A level work else where.
Make certain you find a mate/spouse/etc that enjoys moving every 2 years or so. The post-Doc period is a period of major stress for every marriage as you move from job to job looking for the Good Life. Also, get used to being in a small profession...there are only 6500 astronomers in this country, and 1/3 of them want your job.
Finally, develop a sense of humor and sardonic wit.