Help me to take photos (portrait) beatifully please and battery status

(1)So I just bought a new vivo x300 (standard ver) and got so excited about its camera. But i absolutely know nothing about photography so what should i learn to prepare for my upcoming cosplay fes trip? I want to take some portraits of other cosplayers.

(2) So do any vivo's battery will be unstable in 5-7 days after activating? Mine is kinda unstable when the 3rd day the battery just falls even faster than normal.

Thanks for the help!