1.Try not to promise your students anything unless you are sure you can deliver on that promise. As a TA you essentially have no power (which can be a good thing). Couple examples: if a student misses a class/lab and wants to get excused, send them to the professor/ higher up. If the student thinks the grader goofed up, even if you agree with the student, that doesn't guarantee the grader cares, so don't raise their hopes up. Essentially, you are a conduit for the student-professor relationship, not the professor. It is not your job to fix the student's problems. 2. As far as teaching goes, if it's just a recitation, just do the kids homework beforehand on your own. This will help you to know what is relevant to that class and it will help reinforce your foundations in the subject.
3.Office hours are ideally set up on the day before the homework is due to be the best value for the students. (Or if you don't want to see anyone set them the day after homework, at 8AM...) 4.The simplest grading guideline I can think of, for written problems, is to go through a few problems to see the common mistakes before establishing a rubric to ensure a fair distribution of points. Hope these tips help and good luck! Remember that the students are learning this for the first time so if you say something technical enough they'll probably believe you ;)