Start with the notes. Learning musical notes and how they appear on sheet music is essential when practicing music theory. The staff is the set of lines on which the notes appear, and once you learn the notes you can learn a musical scale. A major scale consists of eight notes played in combination with each other. Tones and semitones work together to form a major scale. A semitone is the distance between two like notes; if you play the D note, the 12 notes played until the next D note is the semitone. The gap between two keys is the tone; when you play a white note/black note/white note combination twice, it is the tone. 8notes.com has information about scales and practice scales for learning music theory.
Take a quiz to check your progress. Taking a quiz shows you where you are solid in your knowledge and where you need more practice. Timed and un-timed quizzes reveal how quickly you answer questions and how well you know a particular area in music theory. See Music Tech Teacher for a quiz that tests the music timing knowledge.
Create a personalized music exercise. Decide what your strengths and weakness are; whether it is identifying keys, understanding tones or identifying different chords. If you have problems in one or more areas consider a tailored music exercise. When you identify your weaknesses you can create a program that helps you address them. Musictheory.net has many personalized music exercises.