Don't really know where to start!?

I just got home from college for the summer and I have SO much motivation, just don't really know where to start! I sadly succumbed to the freshman 15 and am currently around 155 and am 5'6... I want to lose about 15 pounds by the end of August and am dedicating my summer to get my body back in shape!

Does anyone have any advice on any simple workout plans to follow? For now I just go to the gym and do cardio for about 30 minutes then do a few machines but I don't feel as if I am pushing myself enough. I really want to focus on toning my stomach area and arms.

I am pretty good when it comes to eating but would it be worth it to start taking protein? Any advice would be extremely helpful thank you!