Stand up straight with your shoulders level and your eyes pointing straight ahead. Good posture helps you to utilize your abdominal muscles, which is especially important when supporting higher notes.
Breathe deeply before you sing and focus on contracting your abdominal muscles slightly as you exhale during singing. Breathing deeply and using these muscles will help you to support your tone and not "wobble" vocally.
Drop your jaw and relax the muscles in and around your mouth, face and neck. Tension in these muscles can cause you to fall short of the high notes that you are attempting to sing.
Warm up gently. Sing at half volume at first and vocalize a wide range of high and low notes. You can use a syllable like "la" or "ma" as you sing various scales. Start with notes that are comfortable and easy to sing and gradually work up to higher notes.