Welcome to Weight Loss Fitness
Hi there; welcome to your diary and I'm glad you came to this forum where there are loads of supportive people.
You're right to care about your lifestyle and at age 21, you have the promise of a happy future where you're in control.
Most of us have tried different diets, only to gain weight back so don't beat yourself up about gaining your weight back. Instead, you are taking responsibility and that's great!
One of the first things when losing weight sensibly, is to find out how many calories you need for your current weight. I use as a starting point. Then you can subtract 500-750 calories to put yourself in a deficit. Weight loss is pure physics. Less energy going in (calorie reduction) and more energy going out (exercise). You will go through a learning curve about food and it's caloric content, but you have lots of time..be patient with yourself.
Many people here use to log food every day. I use a scale for meat and cheese portions. And I try to stay on the outside aisles in the grocery store for food. You can maybe start reading a book or two on nutrition so you can learn about protein, healthy complex (versus simple) carbohydrates (veggies) and healthy fat (essential fatty acids). Learning takes time and give yourself that time. You don't need to know everything right off the bat.
You can start walking every day. No exceptions. Walk every day. Get a good pair of shoes.
You might also find a support group, besides this online forum, to help you stay on target. Your situation is serious, given the heart disease that is hereditary in your family.
Converting to a healthy lifestyle is a life long process, not something that happens overnight. Don't expect quick weight loss like Atkins. Be patient. Reading the diaries on this forum will help you see others who are losing the same kind of weight you are. Some are at the beginning stages and others have lost a lot of weight. Reading what they've gone through and what they eat will help you. And share honestly what's happening for you, because no one judges here...it's a great bunch of supportive people and lots your age too.
Hugs and welcome.