I Weigh 287.2 Lbs. , I Am A 43 Year

I weigh 287.2 lbs. , I am a 43 year old female, and am exhibiting pain in my lower back. Everything I researched said I should consider losing weight. I started exercising two weeks ago, three times a week (about 20 minutes). Unfortunately, I see no weight loss. Due to these health concerns, I am looking for information on how many calories can I consume in a day?I have researched and have gotten various information. Can you clarify?