I don't think you need the advice of a professional rider at this junction. It would be best to avoid hills if at all possible, and even avoid shifting as much as possible, staying on flat terrain to start, and keep in one gear. The aerobic workout will come from the time spent riding, pedaling at a steady rate for your whole ride, maybe 30min. to start and working up to one hour.
Keep the bike in a gear that you aren't pedalling so fast you get out of breath, and that isn't so slow your legs hurt. Go on an out-and-back route if you have to, flat to easy (real easy) rollers with not much traffic, car or pedestrian to worry about. And get out on the bike at least 4 times a week, if not 5 times, up to one hour each time to get any aerobic benefit.
Then, after a few months (depending on you), ride longer. And eventually save for a better bike, get cycling clothing, and when you can't stay off the bike, seek the advice of better riders.
Eventually you will learn what works for you. Become perceptive to your body as you ride and you will become more confident.
Stay Hungry.
S