I'm trying to learn Mandarin

Hi I'm 13 years old and I am half Chinese and half English. I'm a native English speaker and have lived in England my whole life and in my younger years I wasn't able to balance both languages and then in the past years I simply 'haven't been bothered to learn'. But I'm going to China next year to see my mum's relatives who live there. I now feel dedicated to learn mandarin so I can communicate with my family in China. I think I want to reach fluency at some point. Does anybody know if it's easier to learn at 13 then when u have reached adulthood? And how long will this process take to learn Mandarin? Also where do I start with learning Mandarin? My mum is willing to help but she can't always be there to help me so I'll be able to have her assistance half the time. What should I do to learn?