Pssst... Come here and I'll tell you a secret...
When most of us started, we had no clue what we were doing.
Most important is to start doing *something*. Try 5 min on one cardio machine, if only to boost your confidence! Then try another one. If you're tired, go on the treadmill on an easy speed, just to keep moving--part of the challenge of exercising is just getting in the habit of doing it (which is also partly why it gets easier with time). If the treadmill speed it starts with is too easy, you can try increasing the incline to 1% or 2%, just to see what it's like. Eventually, you'll figure out what machine/exercise you enjoy, and you can look into how to make that more challenging as you get fitter and it becomes easier. If you are intrigued by the weight machines, there are many ladies here who can give you great advice--we have a sub-forum for weights too!
The bikes are totally enough to get a workout. Once you're comfortable with the bikes, they probably have some pre-programmed settings that you can choose from to challenge yourself. I bike to work, and have done so 4 days a week or so since the spring, and I still climbed off a stationary bike earlier in the week and was dripping with sweat.
One thing to consider down the road is that your body gets used to doing the same exercise over and over again, whether biking, running, elliptical, whatever--biking to work isn't challenging for me anymore, so I have to do other things as well to keep my body guessing. But biking outdoors is still my favourite exercise.
The stairs after working out are a challenge, I have to admit. Maybe consider them the last part of your workout? In the beginning, make sure you don't work out enough that you're totally exhausted and you still have a little bit of energy left. And treat the stairs like exercise--one step at a time and you'll get there, and eventually, you'll be hopping up the last flight.
I've heard it said before, and it's totally true--the hardest part about exercising is the 15 feet from my bedroom to the front door.