There are many online tests to determine wether you are primarily an auditory, visual, or tactile learner. Once you know that there are many sources and ideas to study for your specific style of learning. Definitely one of the most important things is to study at the right time and place. Eliminate as many distractions as possible (TV and cell phones are a really big cause for distraction) and do what ever helps you concentrate. It might just take studying an hour earlier to make a big improvement. Don't listen to music with words when you study though! It activates a different part of your brain which limits your focus.
Highlight the stuff you don't know. Pick out the five most important facts on each subject/chapter and memorize those first. Use made up sentences for a multi answer type question. For example to remember the planets, you might use (My Very Educated Mother Must Served Us Nine Peas.) Mercury Venus, etc.