Well, there are lots of different ways you could approach your workouts. If you want to preserve muscle mass, weight training and taking in enough protein are both going to be essential. If you want to lose weight and keep it off for the long term, consistency is going to be key (aka - not 'falling off the wagon' for months at a time).
The current ACSM recommendations are that you should be doing weight training 3 days per week, and ideally you should cover all of the major muscle groups of the body. Start with light weight until you have proper form with each exercise, then very gradually increase the weight. 2-3 sets of 8-12 repetitions of each exercise is a good place to aim for for now, but you might even start off with one set of 8-12 reps just to get the routine established and get used to the exercises.
For aerobic exercise ('cardio'), the minimum recommendation is 150 mins per week... For weight loss, it's more like 250-300 mins per week. That works out to 60 minutes, 5 days per week, or however you want to spread that out, but you want to spread it over as many days as possible, while still giving your body a rest one or two days per week.
I know that sounds like a lot of exercise (and it is), but you don't necessarily start there... it may take a few months to work up to this amount. To prevent injury or burnout, it might be wise to start with 3 days per week doing 30 mins of cardio and 2 days per week doing weight training, then gradually build up the amount of exercise you do each week. It might be a good idea to not always go on the treadmill as well - if your ankles, knees, and hips aren't used to all that walking, then definitely mix it up and try other machines like the bike or elliptical that are a bit easier on the joints.
What weight machines does your gym have? Are there free weights?