How do I become a better competitive swimmer?

Ok, I'm a HS Sopre in the Swim Team. I'm 6'0 and 138 pounds. I'm very skinny. 18.7 Body M Index. My races that I usually do are the 200 IM, the 100 Back, and the 200 Freestyle Relay. When I do the 200 IM i am tired after doing the erfly and troke and end up coming in last place. How can I get less tired and get more energy spontaneously because the 200 Free Relay and 100 back are right next to each other in the line-up. How can I get fatter the healthy way so I can get more muscle. How do I get better in all strokes. And please post your strategies, tips, techniques, workout plans, etc. because I will greatly appreciate it. I want to be fast. I want to be champion of the world and devastate the world.