Losing weight need help

I’m 18 and I weigh 256lbs and I want to weigh 156 I can fit my clothes there just get tight and I start college in August I want to ask what are somethings I can do and what is a way to get ride of the stretch marks I have

This was from prom

I just want a good size that I can be more comfortable in

In the blue dress I can see the stomach more