Congrats and good luck pursuing your interests. Here are some points, mostly focused on the early career aspect (which you can start thinking about as an undergrad):
1) Try to get an internship and get involved with research. Sometimes you will have to create opportunities for yourself, like asking to volunteer at your local observatory. Find ways to get involved with the field, even if they are not in an "official" capacity.
2) Whatever you end up doing for internship/research, ask as many questions as possible. You're new to the field so little is expected, therefore you have nothing to lose by asking and soaking in as much as possible. Don't be afraid of being embarrassed. That being said, when given a task or set of instructions, try your best to figure it out first. In other words, there's a fine line between asking useful/good/relevant questions and appearing (or behaving) completely inept.
3) In an internship or working environment, try to do extra and make a good impression. Partly, your goal is to build a network of contacts and people you can learn from and who will speak well of you. Don't burn bridges.
4) I'm not sure of other tips, beyond the basics of studying and keeping on top of your academics. PhD programs and careers are competitive and everyone else will have excellent grades, etc.
Regarding what I wish I knew...maybe that getting good grades and "being smart" are not the equivalent of attaining knowledge. I always did very well in classes, etc., but in grad school I realized that I really needed to UNDERSTAND everything at a deep level. Also, to be successful in academia, you have to be inherently interested in what you are learning. I guess I wish I practiced more purposeful and interest-driven learning as an undergrad.
