Although you will have done a lot of work over the summer, you will finally start getting results this semester. Of course, the results will not be what you (or your advisor) expected. Thus starts the process of scientific research. Make a hypothesis (what you expect), run some experiments, analyze the results, modify the hypothesis or experiment, and run more experiments. In designing software/hardware, the design process has a similar cycle of specification and analysis.
If you are doing really well you may want to start writing an outline of your work in the appropriate (thesis or professional paper) format. You will also be looking for good conferences or journals in which to publish results. We now expect students to give a departmental colloquium explaining what you have done and the problems you face. You and your advisor will schedule the colloquium in conjunction with the departmental colloquium coordinator. A colloquium is good practice for an oral defense and the audience provides good feedback. Attending colloquia is a great way of learning how to do it and what kind of question to expect during your defense. It is also fun to learn what others are doing, how your work may relate, and bounce ideas off each other.
This semester is also a good time to finalize a graduate committee to whom you will defend your work. The committee is made up of your advisor, one other faculty member from the department, and one external faculty member. The external faculty member is someone from another department.