Here are some tips that have always helped me:
-Breathe from your diaphragm. To find out if you're doing this, put your hand on your belly button. Breathe in slowly. If your stomach pushes your hand out, you're breathing from your diaphragm already. If your stomach doesn't move or goes in, you aren't breathing correctly. To breathe correctly, keep your shoulders down and breathe in slowly, pushing your stomach out as you do. Although it may feel uncomfortable at first, after a while and a lot of practicing it will feel almost natural!
-Before you sing, always warm up so you won't damage your vocal cords by suddenly blasting out a note. Try singing "Ah" starting at the top of your range, and sliding down to the bottom of your range. When you get to the bottom of your range, you will hear that gravelly sound you usually hear when a note is too low for you. This sound is created because there isn't enough air between your vocal cords to create a regular sound. Slide back up from the bottom of your range to the top of your range, and repeat this exercise until you've eliminated the gravelly sound at the bottom of your range. This is a great singing and breathing exercise!
-Drink flavored water, as opposed to regular water. I know you've probably heard that regular water is the best thing for you about a million times, but this is one of the best tips I've ever gotten, and it's helped me immensely. Flavored water is so much better because if you haven't noticed, when you drink regular water it quenches your thirst, but in about a minute your throat is actually feeling even drier than before. This is because the water just washed the wet lining of your throat. Flavored water quenches your thirst and has sugar to coat your throat and keep your your throat in good condition. I know it may sound a little bit silly, but it really does work!
-You always want to tell a story with your song, especially in musical theater. The best way to do that is by first speaking the entire song as if it is a monologue, and using the emotions you think are fitting to the words. Then, sing the song with those emotions and expressions in mind. You'll notice a definite improvement.
-Another way to help you to tell a story is to picture the scene you're describing in your song. Try and relate the lyrics to things that have happened in your life, and think about how you felt and what you looked like (expression-wise) when that happened.
-If you are struggling to hit a high note that's almost out of your range, use this exercise to ease yourself into the note, which is healthier for your vocal cords and is much easier than just singing a high note straight out. Position your mouth as if you are about to eat an apple. When you go for that high note, open your mouth wider and longer, and move your mouth forward as if you're biting into an apple.
I luv singing!