If you're not doing well in a course, you'd normally want to talk to the instructor or perhaps your departmental advisor. Lately, however, overcrowding has made this hard to do. So here are the most standard sorts of advice we'd give you in person, to at least get the ball rolling.
How badly are you really doing? In high school, you were one of the top couple students and you could get all A's. However, everyone in our CS program was a top student in high school. A B is not a bad grade. If your high school preparation was iffy (e.g. your high school wasn't the best), then you should worry even less about a B in this class.
In this program, a reasonable goal is to get A's in the courses that most interest you, especially your upper-level elective courses and projects. No one is very good at everything. An employer or graduate school will want you to show that you're very good at something. Sure, a future theoretician should have an A in CS 173, but a future prize-winning hardware designer might not. Your first couple years in college should be about finding the area of CS that suits you.
If you need to take CS 374 (i.e. CS and CE majors), a grade below B- in CS 173 could be a real worry. Finish the term as well as you can, but also speak to your departmental advisor about whether to repeat the class or take other theoretical classes (e.g. statistics) between CS 173 and CS 374.
Special situations:
Do any of these seem to characterize your situation? (More than one might apply.)