Ok, the first thing you need is support. Your diaphragm (muscle) is responsible for your exhale and thus, your singing voice, and also all your muscles of your abdominals internal and external, plus the pelvic floor. If you act like you are crying, feel your stomach. see how it tightens and supports your cry? This is what you need to do to reach those strong, high notes and to be able to have a voice after hours of singing. Never let your support go. Sometimes you need to push down like you are going to go to the bathroom, number two. (Don't think it's crazy, it's true!)
Breath. You need to be able to control your breath or singing will sound funny and choppy. Take a few deep breaths and then exhale slowly, controlling your exhale. Now bend over slightly, put your hands on your knees, and inhale as much as you can through your mouth. Once you feel that you have filled your lungs to their capacity, take a few more small inhales. (Don't hurt yourself!) Now exhale slowly and control it, and once you feel you've exhaled completely, push a few more breaths out. Repeat this 3 times. (This can make you dizzy, so make sure you are near a couch or bed in case you need to sit down.)Make sure you don't raise your shoulders when you do this, keep your shoulders down. This breath exercise helps to increase your lung capacity and helps you learn how to control your breath.
Vocal warm ups/exercises. You should warm up before you sing so you can last without straining or damaging your vocal cords. I recommend at least 15 minutes of warm ups. For a warm up, start with a hum. This is a great way to get your vocal cords warm and relaxed. Start in a low key, hum a sustained note, then inhale through a small open mouth (too wide and your throat will dry out) then hum a little higher until you've reached your vocal range limit. Hum scales like do re me fa so la ti do. If you have a piano you can find the notes on it. If you don't just hum as best you can.
Next, lip bubble. This is where you make your lips vibrate while you exhale and make sound. This helps to teach control while reducing strain to the vocal cords. To help your lips vibrate, take your thumb and your middle finger from one hand and press the corners of your mouth gently together while you do scales. Now, I want you to do these scales softly. Start low with a do re me fa so la ti do scale and move your way up as high as you can go.
Now you can do the muh. Yes, this can be compared to a vocal fry. It's where you're acting like your upset and almost crying. Sort of whining. Make sure you are doing this softly. So, go muh (really sad, now a little higher) muh (now a little higher, make it sad) muh (now a little higher) muh, and now go back down. Make sure you are really whiney and do this going up in scales as high as you feel you can and scales down as low as you can.
Now you need to sing. You need to sing for about an hour straight every day so you can build up your endurance for a 1, 2, 3 or 4 hour gig. Sing whatever it is you love. Clean the house and sing. When you drive your car sing. Sing all the time. I heard an interview with Mariah Carey once and she said that she sings every day, so you must sing every day too. After a few times of doing these tips you will notice how much better and stronger your voice is. Keep it up and let me know how it goes. Soon I will be making a lesson on CD so check back once in awhile. Write me if you need any tips.