You learned everything you know in three different ways: through your eyes, your ears or by doing/touching/tasting. You probably learn best through one or two of these ways, making you a visual, auditory or kinesthetic learner. Finding out which is your learning style can help you use it to your advantage to play to your strengths.

[edit] Steps

Find your style. Do you learn a song by listening to it, by reading the lyrics/sheet music or by playing/singing it? Experiment with all three.

[edit] Visual learners

Read the textbook (or your notes). Highlight the key words.
Draw lots of diagrams, if possible on a large piece of posterboard. Link concepts with arrows.
Use colors to highlight similar concepts. For instance, in History use different colors for economic, social and political events.
Watch videos on the subject.
Make flash cards.
To learn vocabulary words (for a Foreign Language class), write the words down on Post-It notes and then stick them on the corresponding objects so you’ll see them throughout the day. If not possible, make a drawing and then stick the Post-It on it. To review, unstick all the Post-Its and try placing them again correctly.

[edit] Auditory learners

Pay extra attention in class to what the teacher says. Make an effort, it’ll pay.
Read out loud from the book. Tape yourself.
Movies help you also.
For a foreign language, tape yourself reading vocabulary words. Call a friend and have a conversation in that language.

[edit] Kinesthetic learners

Take notes during class.
Do something –anything— with the text besides just reading it. Take notes, copy key sentences, read out loud, whatever.
If your school has a lab available, use it.
For history, place action figures on a map and move them around accordingly.
For a foreign language, act out/touch/taste the vocabulary word. Stage a small play with a few friends using these objects/actions.
Act it out! The most obvious subjects that can be studied this way are history and literature, but with a little imagination you can act anything out. Grab a few friends be the organs in the respiratory system, or the parts of an atom.