First things first: make sure your finances are in order.
As you already know, graduate school can cost a lot more than undergraduate school, even if it takes less time. Make sure you have a plan to pay for the entirety of your program, because you don't want to make it halfway through and realize you don't have enough money to finish.
Check with the financial aid office so that you understand all of your options. Millions of dollars in scholarships go unclaimed every year, so be sure that you apply for all of the scholarships you can qualify for. You may also qualify for student loans, but keep in mind that your options are less limited than undergraduate loans and may costing you more in the long run.
Internships, Teaching Assistantships, and Research Assistantships.
Student loans have to be paid off eventually, and there is just no easy way to get out of them aside from student loan forgiveness programs, which you should check into after you graduate. However, as you are getting your degree, check into paid internships, teaching assistantships, and/or research assistantships.
Depending on your program, you may be able to make some money while getting graduate credit and gaining on-the-job work experience. This experience will pay off tenfold after graduation. Employers love to see recent graduates who already have experience working in the field they are studying.
Graduate school can be significantly more difficult and take a lot more of your time than you expect. Be sure that you are mentally ready for the work involved. Do some reflection on your study habits in your undergraduate and what it took to be successful.
Likewise, each semester, look over your syllabi and plan out your entire semester in terms of reading, assignments, and exams. It doesn't have to be a definite plan, and it is ok to deviate from your plan, but you'll find that looking over things in advance will ensure you are caught unaware. Time management is essential if you want to be successful.
One of the best things you can do to get involved is to network with your professors. You will likely have to assign an advisor either at the beginning of your schooling or at the end for your thesis project, so you will want to know who will be the best fit for you. Choose someone who aligns with your own specialties and areas of interest. Professors will tend to become lifelong mentors.
Also, meet with other graduate students in your program and form a cohort. Surrounding yourself with other graduate students is a great way to keep each other on track, form study groups, and succeed as a whole. Other students who are further along in the program will also help you choose the right professors and help you become aware of extracurricular activities.
Don't stress, but also don't get lazy.
Stress is the number one enemy to being a good student, which means you need to plan time away from your studies to unwind. Don't get lazy or procrastinate, but also realize that focusing purely on academics isn't beneficial to learning.
As you begin your graduate program, keep focus and stay prepared. You got accepted, so you are obviously a good student. You're now only a couple of years away from obtaining a career in a field you want to work in and begin a purposeful life.