Familiarize yourself with dynamic marking letters. The three basic markings are "p" for "piano" (soft), "m" for "mezzo" (medium) and "f" for "forte" (loud). These markings will appear below music, indicating that the notes below are to be played at this volume until otherwise noted. "M" will appear along with "p" or "f," indicating that the music is to be played at a medium soft or medium loud volume. "P" or "F" by be doubled, tripled or quadrupled to indicate larger extremes of loud of soft.
Learn to recognize other dynamic symbols. The music may be marked with the word "crescendo," meaning to grow louder gradually, or "decrescendo," meaning to soften gradually. These words may be abbreviated as "decresc." or "cresc." Alternately, a symbol like a long, large "less than" sign (<) indicates a crescendo, while a similar version of the "greater than" sign (>) means to descrescendo.
Practice to develop the dyanamic extremes of your instrument. During each daily practice session, spend a little time playing at the softest and loudest you can play. Work to expand these abilities in both directions, making your loudest sounds louder and softest sounds softer.
Develop your dynamic control. Practice playing passages at varying volumes and practice decrescendos and crescendos at varying speeds of change and of varying dynamic ranges. Work on being able to play at a consistent volume for long stretches as well.
Match your dynamics with the people you play with in ensembles. For optimal matching, your sound should be a little louder in your ears than those around you, since it will be the loudest to you. Pay attention to what others say to you about your sound and how the volume measures up, or make recordings during rehearsals and listen back to see how your sound balances.