Warm up your vocal cords for at least 10 minutes with scales and other vocal exercises. Each time you practice, expand your range by one note if you can.
Let out a loud shout, like you would use if you were stopping a child from running across a busy street. Now shout again, but this time hold the tone, extending it into a note.
Start singing by taking deep breaths using your whole chest. Don't push from the throat, which can damage your vocal cords and doesn't sound very pretty. High notes require more breath pressure to achieve and sustain the desirable sound. If a vibrato comes into play, consider that a bonus that will help you sustain the note.
Hear the high note played on the piano or on a recording, then try to replicate it.
Understand a little about the mechanics of singing. As you go up the scale, you will use your chest, head and pharynx to adjust the sound. You will raise the soft palate in the back of your throat to reach the high notes, but it's not something singers generally think about.
Use psychology to combat fear of the high note. Visualize yourself standing on a ladder and looking down at the note, or picture yourself riding up an elevator and each note is a "floor." For high notes, you might have to shoot that elevator through the roof.
Spend time with a vocal teacher who can help you expand your range to the upper register. She can help you reach your potential with exercises and songs that you can master and improve your range.