What's the best way to start exercising again?

I use to go to the gym everyday (minus Sundays) and work out for about 1 hour (treadmill or bike or elliptical and a few weights). Due to changes in my schedule I just got out of that habit and haven’t been to the gym in the past 3 months, and this past week while going up the stairs of a building (I just went up one floor) I found myself out of breath – it was kinda scary, it took me about 30 seconds before I could breath normal. What exercise and how long would you suggest for me to get back in shape? Should I just go back to what I was doing before or do you have any different ideas? I need help :)