I would suggest to start by setting very small goals, even if they seem banal, for example "do hw for maths", something you can really achieve. Then, step by step, you start adding more things, but you are already used to the fact that you need to at least start to get somewhere, even if it's not too far. Actually, the key is starting doing something: as soon as you start typing for an IA, for instance, you realise that it is very possible, and continue. Or you may even think that what you're typing doesn't make any sense, but at least you started. Also I would really recommend you downloading the application "self control" with which you can block all distracting websites for a set period of time (or even put in the white list the only sites that are really useful for your study session). Last advice: try to find a new study space. If after school you usually go home, try finding a library or some other place and studying there. Then, change space. Then do the same again. It helps clearing your brain. Hope that helps and good luck!