Well, you may need to adhere to some form or another of a schedule, even if you don't like them. What about setting a reminder on your phone, to put it away after 30 minutes or whatever? There are ways to get work done on a computer without being distracted by the Internet (e. g. don't open a browser window).
You might also want to operate on a small rewards system. That is, you get an hour's worth of homework done or three pages written of a paper or memorizing a musical piece or whatever unit is substantial and makes sense to you, and reward yourself with 15 minutes of phone time and use a timer so that you don't go over.
Kids in High School often have time management issues, so you're not alone, but you'll need to do some prioritizing. Maybe even work with your folks on this, and develop incentives if you can keep it together. Whatever those positive incentives are, make them something that your folks are willing to give you, and you want, but they're not so easy to achieve. An incentive might be a later curfew for a month, or an iTunes gift card, or whatever appeals to you. But these bigger rewards would have to be earned after some heavier lifting by you. An hour of doing uninterrupted homework shouldn't be enough for these, but maybe an hour of doing uninterrupted homework for a month might make sense and be proportionate.