You may have some dietary limitations from the colitis, so I first would recommend that you ask your doctor to refer you to a dietitian. I believe that there also is a Team here on Spark for that condition, so you might want to join it so that you can chat with others who are dealing with the same thing.
As for exercise, I'm not aware of any limitations on that due to colitis, but please check that with your doc, too. The main thing about fitness is to find things that you really enjoy, and to include both cardio and strength. Walking is a great way to start, a lot of folks like Zumba or other dance-based classes, and strength training can start with body weight exercises and resistance bands. There are a lot of exercise videos here on Spark, as well as elsewhere on the web, that you can try out to see what you enjoy.
Sorry to hear of all that you've been through, but major kudos for taking this step in taking control of your life!