What is the best routine to do to get back into regular exercise?

I just got back from a gym class that I use to take a few times a week regularly about a year ago. Not even half way through, I was feeling sick and dizzy and incredibly weak! I can’t believe it!

I am finishing my final year at University and I neglected my gym classes due to all the stress and assignments, but I want to get back into the gym now.

I am quite thin and I have lost about 8–10 lbs since I use to attend classes regularly. I think that the problem today was simply that I jumped into a class that was too intense for someone who hasn’t pushed themselves fitness-wise for nearly a year. The classes at my gym are all fairly intense, so I think the best thing to do is to do some training alone on the cardio and weight machines so I can build up stamina.

Could anyone recommend how long my cardio sessions should be and how much weight I should be doing each session to get back into the swing of things, or even some general tips for getting my energy up for a workout?

(I do eat very healthy already, fruits, vegetables, lean meats and lots of water.)