Kami, sounds like you're ready to start so although I'm not an expert here are a few suggestions; first is to calculate your cardiovascular heart rate using the following formula (220 minus your age X 75%). Since you are 23 this works out as follows; 220 - 23 = 197 X 75% = 148 BPM; second, obtain a heart monitor which will display the number of beats per minute as you exercise; third, begin exercising moderately and for short periods of time, no more than 5 minutes the first time, increase by 1 minute a day as long as you feel good while doing it. In 20 days you can be walking, riding, or whatever for 20 minutes and you can keep on going up from there.

Don't overdo it, you gained the weight over time and you must lose it the same way. Moderately and with patience. The heart is a muscle and this exercise will burn calories and strengthen your heart. The more you do the more you can do. You will see your heart rate decrease and you will be able to do more. If possible, have one session with a personal trainer (I know they charge by the hour) it would be a good way to get started. I hope this helps.