I'm a sprint freestyler, but I do know exercises for fly. Upper chest exercises like incline bench are great for overall strength in your stroke. Lat pull downs and wide grip pull ups increase power through the pull. Squats can make your kick more powerful. Fly mainly uses chest, shoulders, lats, glutes, hamstrings, and quads. more reps with less weight will increase endurance, less reps with more weight will increase strength. If you primarily do the 100 and 200 medley relay I suggest you do less reps at a higher weight, and if you do the 200 vice versa.
Biggest point to remember as you start lifting: as a swimmer, you need to stay flexible. Stretch every day to stay hyper-mobile and maintain flexibility. More range of motion = bigger pull.
If you want a structured personalized workout program designed for swimmers, Nick Folker was the Cal Berkeley swimming weight coach (coaches Nathan Adrian) and is now creating workout programs for people through BridgeAthletic.
Good luck and be safe.