First you should really consider if you want to go to Princeton and why because there is a possibility you may not like it, but if you are determined in going there that's fine.
(I am assuming you are in High School) You need to check out their average SAT, ACT, and GPA's so you get an idea of what you have to achieve in order to gain admission (if in college same thing except GRE, GMAT, MCAT, LSAT etc.)
SAT: 2100 - 2370 CR - 690 to 790 M - 700 to 790 W- 710 - 790
GPA: (not for sure on this one) anywhere above a 3.8 gives you a good shot, a 4.0 is probably your best bet to get into princeton though
Also its good if you take 2-3 SAT subject tests i.e Chem, Math II, or History
The PSAT is not a blowoff btw, if you do well on that you have chances for merit scholarships and it does help in gaining admission.
Make sure you take challenging classes i.e AP classes whenever you can over taking on-level classes because it shows that you challenge yourself.
Some good extracurriculars that demonstrate your work ethic and who you are. It also shows that you can balance your school life and work outside of the classroom. Organizations like NHS, Student Council etc. are a good way to get in. (Usually organizations with some prestige will help even sports like B-ball)
Your essays can be a deciding factor in your application. If you have a good strong essay that is thoroughly done it may put you in a better position.
And if you are definite that you want to go to Princeton over every university out there, then applying early decision gives you a better chance of getting in over regular decision. baiscally its a binding contract that says if your accepted then you will go and generally the acceptance rate for early decision is greater. For instance Yale's regular decision acceptance rate is close to 5% while ED rate is around 20%.
This is just some basic stuff you need to do, but really you need to some extra research see what other students who have gone there did etc. I also suggest you take a few campus visits so get an idea of what the school is really like. You always need a backup school however, so don't be so fixated on getting into princeton when you really need to focus on getting in so just keep that in mind.
Getting into Princeton is tough, though not impossible. There are so many well qualified students with similar scores etc. that get in. You need to make yourself standout that is probably the most important thing.
My brother goes to Princeton, I applied got accepted (don't go though not my cup of tea)