How much is an adults ticket to london by tube from elm park?

i know nothing about trains n my sisters forcefully asking me and my mother to travel to some arch bridge place whatever in london but how much roughly does it cost for an adults ticket and a ticket for 15/16 yr old??? lmao i don't even know where to get the tickets from also my mother has an oyster card and i was wondering whether she could put money on that idk help this is my first time tryna get round these kinda places lmao