The first step is to get rid of any kind of distraction. This can range from small things, like cleaning up a cluttered desk. A cluttered desk may indicate the disorder of your thoughts - so clearing it up might just help in sharpening your focus.
If you are working on your computer for a project or researching a topic you need to study, be sure not to have unnecessary programs running. Turn off your TV and your stereo. You need to concentrate. Of course, if you feel that you can work best while listening to music, then leave it on - but softly, so that it's more in the background.
Another great distraction in this age of technology is the ring of your cell phone. Phone calls and text messages from friends should not be entertained. Put your cell phone on silent mode, or even better switch it off. Tell your parents to take a message if there is a call on the land line and to tell the caller that you'll get back later.
Once you have got rid of the things that may potentially distract you, set targets for yourself on a day to day basis. Take five minutes every morning to set yourself a goal for the day on what you need to accomplish. Writing a plan for the day may be helpful for some. This doesn't mean writing everything out in great detail - just outline a basic structure of how you are going to achieve your goal in clear terms.
Breaks and rewards are essential. Take a break every three hours or so to refresh yourself and your brain.
If you achieve your target for the day then give yourself a treat to look forward to. Plan again for the next day and let this one go.