Need help with Realistic Goals

Start here:
http://www.myfitnesspal.com/topics/show/1080242-a-guide-to-get-you-started-on-your-path-to-sexypants

And here:
http://www.myfitnesspal.com/topics/show/819055-setting-your-calorie-and-macro-targets

I think taking control and deciding to make a lifestyle change is an awesome first step toward reaching your goals. You've asked a lot of questions and it looks like you want to start jumping right into everything at once, which is awesome, but don't overdo it! You can take this one step at a time and avoid feeling burned out and exhausted. You're about to get a lot of good advice from a lot of experienced MFP users and you're also about to get a lot of crazy advice from a lot of inexperienced people. My advice is to take everything everybody says (including me!) with a grain of salt and do your own research.

I suggest starting out by figuring out an estimate of how many calories your body is using now at 281lbs to stay up and running with your current lifestyle. Are you sedentary, active, SUPER active? These are important things to keep in mind.

You can put that information into this: http://scoobysworkshop.com/calorie-calculator/

Once you've figured out how many calories your current body needs, you can reduce that by a percentage (15% or 20%) and your body will be placed in a caloric deficit. You can customize your goals on MFP to match the suggestions you get from Scooby's Workshop.

If your doctor suggested low carb, you should call and ask exactly what they meant by that. Do they mean 5% of your diet? 30%? less than 100g per day? There are a lot of different things that could be considered "low carb" and nobody on this site will know exactly what your doctor meant by that.

As for exercise, keep in mind that MFP calculates a caloric deficit that does NOT include exercise. This means if your goal is 1900 calories per day, you will lose weight without exercise. These are NET calories (total calories your body uses - calories burned by exercise) If you DO exercise, you should eat back at least some of those exercise calories to keep fueling your body. You want to be as close to your goal as possible each day.

For example:

MFP goal is 1900 calories per day and let's say that you DID eat exactly 1900 calories.

Then you exercised really hard and burned 300 calories.

This leaves you at 1600 NET calories (1900-300-1600).

You should eat 300 calories again to lose at the rate you are working toward.

I'm not as familiar with the exercise part of this since I don't exercise right now, so I'll let somebody else cover that.