How can I improve my mobile security?

Hi everyone, A few days back, I was trying to schedule my gynecomastia surgery procedure with my cosmetic surgeon, and my friends kept glancing over at my screen, trying to see what I was doing on my phone. Although I love my friends, they can be a bit too nosey. Later on the same day, I caught them trying to open my phone. I know I have no reason to hide from them, but I am extremely insecure about my body. I want to undergo this surgery in peace. 

I was wondering whether anyone here can successfully tell me ways I can improve the security of my phone in general. I would much appreciate all your help!