At 280 and 5'3 -- just go for losing 10 lbs for now. Once you get there then reassess where you are at, what you are doing etc. when you are ready to hit then next 10.
Have you had a physical with your doc? To make sure you don't have any hidden obstacles like thyroid?
Do you know where you are and how you got there? Keep a food log for a week or 2. See what your eating habits are like with NO CHANGES. Then you can see about subtracting 250 calories from the log.
Then on the fitness front -- look into cardio, weight training, and flexibility. Again keep a log with no changes. Then start tweaking so you burn 250 calories there.
Then you have a daily -500 calories and that's a solid -1 lb a week.
If you want guesstimates with calculators for calorie level, weight, etc. try here. There's several on the left column.
http://www.freedieting.com/tools/calorie_calculator.htm
GL!
A.