Keeping with the theme of being a TA, which I agree has been a valuable experience, I am going to expand on Alexandra’s second point: Time Management. Being a TA at Rutgers commits you to 15 hours of work per week. For my course, this includes 3 hours a week of actual teaching, several hours of planning classes and reading (or re-reading) course material, 1-2 hours of office hours, and a great deal of time spent reading and grading student work and responding to their emails. Generally, this is very manageable. However, there always comes that moment at the end of the semester when you wonder if you will ever get all of your work and grading done. I can tell you that you will; it just takes some careful planning throughout the semester.
My first piece of advice is to work ahead where you can on your own coursework. Yes, that project may not be due until the last day of class, but if you can start it early, you will be much less stressed in the end. Second, if you can get an existing syllabus for the course you will be teaching during the summer before, this gives you the opportunity to read the materials for the course ahead of time, rather than trying to do everything during the semester. Finally, figure out where your students struggle and work with them throughout the semester. My students have to turn in three extensive written projects. I provide them with a large amount of support on the first two projects so that they are able to write the final project independently. Most courses have a final project that is due at the end of the semester, when graduate students are busiest, and helping students extensively throughout the semester means that you can devote more time to your own work at the end of the semester.
While each course is different, these tips will hopefully be useful as you begin your graduate career. If you are offered the opportunity to TA, I highly recommend that you accept the offer. While it does mean some extra work, the benefits are enormous and the workload is certainly manageable.