The answer is right now. The first semester of your senior year is when you should start sending out resumes en masse and meeting with every company you can. This will be part 1 of 2 in a series of how to start looking for a job.
Companies are taking each hire more seriously now, they want more than technical prowess.
Interpersonal skills, good communication skills, the ability to work effectively in teams and demonstrated leadership potential are what companies are looking at more closely in each applicant. Lockheed Martin, for example, has a 2.8 QPA cut-off on who'll get an on campus interview, but work experience and extracurricular activities carry significant weight. These are things that you should put on your resume and elaborate on them when you have the interview. The first thing that you need to start your job hunt is a resume. This piece of paper will either get you an interview or a rejection based on how it looks and what it says.