Advice/suggestions for beginning moderately intense workouts?

About 6 months ago I suddenly decided, or rather just figured out, that I was no longer comfortable being obese - I was 5'8" and weighed 230 pounds, so I decided to start going to the gym every other day, primarily just for ~40 minute cardio sessions. I didn't really change my diet nor my semi-sedentary lifestyle habits, I just did cardio much more often. I've lost about 50 pounds as of late-July, and it seemed (or seems) as if I was hitting a plateau at 180 pounds, so I decided to slightly change my diet - By that I mean I'm just assuring that I eat roughly 2,000 calories every day I don't workout, and about 2,500 calories ever day I do workout. I'm also considering to begin muscle workouts in tandem with my cardio, but I'm not really sure where to start; I can barely do 2-3 reps of 10 pushups.

So, I suppose what I'm looking to receive is some tips/recommendations on where to start, what to do, how often, etc. in regards to muscle group workouts, cardio, and nutritional habits.