What if I get B's in my first semester in graduate school (engineering program)?
Ultimately, your grades as a graduate student matter very little, except for:


A handful of employers who have GPA requirements for graduate programs;
Fellowship programs, which may require a GPA minimum;
The academic regulations, which may also specify a GPA minimum to continue in good standing.


However, it should also be noted that most graduate courses are based on the premise that a grade lower than B is a failing grade. Therefore, it's better to get A's than B's in graduate classes. But, at the end of the day, having B's is probably not going to impede your career much—especially once you enter the working world.