I would agree with Hooman, that is pretty sound advice.
Another common thing that I have always been told is "for every hour you spend in a class you should spend 2-3 hours outside of it studying or preparing for it." While this doesn't really apply to high school (at least where I went to high school, in the US, you probably have anywhere between 6 or 8 classes every weekday, so there is no way to spend this much time on every class), or classes where you already know the material (and I would agree with that, you don't really "know" the material unless you're capable of teaching it to someone else); for those classes where I really didn't know the material this is important. (FWIW I am a university student and I will have my bachelor's degree in about 5 weeks, so I am no real stranger to studying for classes)
You should develop good study habits. This includes setting aside specific time tp study, where you will be free of distractions (you should also try to study in the same place every time; it's proven that encoding and retrieval of information in memory is best if you do all of the encoding (learning/memorizing/comprehension) in the same setting).
If you are going to be distracted by other people, TV, cell phones, the internet, etc. do whatever you have to in order to make it extremely difficult for these things to distract you (if you have to turn your phone off, and take the battery and SIM card out and put them on opposite sides of the house then so be it. Libraries are great places for preventing you from socializing, watching TV, phones, video games etc. forcing you to actually study).
If you are a type of person who procrastinates on all the studying / completion of term papers / etc till the last minute you have to change this ASAP. While this might be passable in high school and let you scrape by with B's, in college/university this is an excellent way to start getting F's. Devote a certain amount of time each day towards completing the assignments in a timely fashion so you aren't going to be writing 10 pages of term paper the night before it's due. (That's not to say that cramming doesn't happen for exams, indeed it does but that should not be the only time you're studying for an exam).
I think the biggest thing that would help, rather than trying to just "get by" with Bs you should be trying to push yourself and get excellent grades (As and such), not only does it boost your GPA which helps when the college admissions staff are looking at your transcript but also it will prepare you to perform better after high school.