it's normal for a 12 year old like you to choose playing games over studying. You can't force yourself to unlike it. you must have an inspirations. search for "ferrari" on google images. do you want it? then study hard. do you want to be the next bill gates? then study hard. do you want to beat japanese games? then study hard. but since you're 12 its hard so it's up to you.
Also you might wanna check out a tool like https://macfreedom.com/trial which can lock you out of the internet for a set amount of time and you can't reverse it unless you reboot. I found something like this very handy when studying from my books. There is better tools out there that can block certain applications like games and internet, ect but they cost a bit more.
I still use freedom today when I need for force myself to work on a project. I just make sure I have all the documentation I need downloaded on my computer then set freedom to lockout my internet for a few hours and just code.
first thing in the morning is good because the details of the day would inefere otherwise, also if you have a focusing problem in the UK you can get prescribed ritalin, In the US I think you can buy it out right, if you take that root keep an eye on your weight and paranoia.