Here are a few basic things you'll need to know even before you set eyes on your first paper to grade:
Be Fair, Objective, and Prompt
In other words, grade the way you'd like to be graded!
Grading on the Curve vs. Grading Against a Preset Standard
Decide on your "strategy" for giving meaningful grades to your students.
University, Departmental, and Course Policies
Make sure you know the policies that are specific to your department/ course, since you'll have to implement them. Be aware of them before you break them!
Student Privacy (FERPA)
To put it simply: only the student in question is supposed to know his or her grade. So don't post the grades on your door, don't make them public, don't announce the grades in front of everyone, don't show the papers to the whole class.