The mind can absorb only a certain amount of new material at a time. You can't learn everything about everything. The solution, then, is to be selective. Choose what's important. Learn the important things and then build on that knowledge (basic background).
Here are some tips in choosing what's important.
Look for clues when reading a textbook assignment. Use a survey method before you begin. Look at headings, graphics, and bold print. Study the summary and review questions before and after you read.
During a lecture, listen for verbal clues such as emphasis and repetition. Pay attention to non-verbal clues such as the lecturer's body language and information written on the board or given as handouts.
Make yourself the test maker. Constantly ask yourself,"If I were giving a test on this material, what would I ask?"
Making flash cards for information you need to learn is an excellent way to employ this principle.