If you want to be a plus size model, you have to make sure you meet the requirements. In a nutshell plus size models are exactly like fashion/runway models except the measurements are larger. For example, plus size models should be 5'8" or taller (there is no demand for shorter plus size models unfortunately). Sizes range from 10-16. Your measurements (bust, waist, hips) should be proportional and about 10 inches in difference: for example 42-32-42 are ideal measurements to have.
Getting a good modeling agency to book you work is the best way to have a good modeling career. Search online for modeling agencies in your city/state that have a plus size modeling division. Their websites will give you all the information you'll need as far as what pictures to send and if they have open casting calls you can attend.