Learn to "belt". If you are a trained soprano or used to singing in a light head voice, you have to rethink where you place the majority of your notes. "Belting" is singing from your "chest voice" rather than "head voice”. This means the placement of each note should feel as if it is coming from your chest or, on higher notes, from the front of your face, as opposed to resonating up in your head.
Listen to Christina as often as possible and notice how she riffs around the melody. A riff is the extra stuff on top of the traditional melody of a song that the singer fits into a phrase of music. If riffing does not come naturally to you, break down each line that she sings as if it were the melody and learn the riff note for note. The more you do this, the more you will be able to create your own riffs.
Support your voice by breathing properly. Lie on the floor and breathe normally. Watch how your stomach rises when you breathe in and falls when you exhale. Natural breath control has nothing to do with your chest or shoulders. Unfortunately, when people sing they often breathe from their chests, which results in shallow breath and can lead to voice damage, especially if you are belting a la Christina. Concentrate on imitating the breath you took lying on the floor. Your stomach and ribs should expand as you take a breath. You will be able to sustain notes longer supporting your voice in this manner.
Hold your notes out! Christina will often hold a big note for a long time. Now that you are supporting properly, you can do the same. Start the note on a straight tone and then hold it out only adding vibrato at the very end of the note. This is the pop style that Christina generally uses for long notes.
Sell it! If you've practiced these steps then Christina's vocal style should be emerging, so now you can focus on selling the song. Christina truly delivers a performance. Focus on the essence of what each song is saying and try to convey that to your audience. Focusing this way should help you stay in the moment as well, which helps with nervousness because you are not thinking about anything but getting your point across to the audience. Now perform your heart out. You are a star!