Jaycel has given you the best answer so far. Warming-up by humming gently or doing lip trills ( also known as raspberries, where you stick out your tongue and splutter at someone) and humming are good starters. Paying attention to how you use your breath is the #1 area of concern for most singers. If we neglect our breathing ( it's the same apparatus we use everyday, we just use it more efficiently for singing) our throat muscles take over and compensate for the lack of power from "below". It's kinda like trying to push your car down the street with the motor off: it can be done, but the wear and tear aren't worth it.
So, the next opportunity you have to enjoy a good belly laugh, notice which muscles are involved in that. All of them, front AND back, are what we want to use for singing. The trick is to get used to breathing like this with every breath you use for singing. Don't get discouraged- it IS a lot of work at first- but practice is what will do it for you.
The #2 area of concern is in the throat itself. Here are two good everyday usages that we can translate into singing habits: sigh, the "OMG, it's the weekend, and I can relax now" kind. You'll feel the air rushing up from your belly ( that's good) and streaming through your throat with a lot of speed but very little power. That feeling of no power/floating on out/ not helping the sound out is very important to maintain. If you ever get the feeling you just have to help your sound out, then you are "pushing" or "forcing" and need to go back to the idea of breath control above.
The other everyday thing is watching how you breath in when you start to yawn. Go ahead! Now, watch that first inrush of air and what your throat and jaw do. Nice and easy and open. Now, inhale again, and STOP the yawn. You stayed open, didn't you? That's h ow open we want to be for singing, especially for higher notes.
So, try these things out, and then take a song you know very well, and try using these ideas, paying more attention to what your body says about how it feels than how it sounds at first.
Since you are still in school, what about joining the chorus? If your teacher is any good, you will encounter these ideas again and again. The practice will be good for you, and you'll be in with people who like to sing. Not a bad bonus!
Best wishes and keep on singing.

Well I'm not going to lie, but no matter what you do to help, the tone of a persons voice is going to stay that way. You can better yourself but the tone will always stay the same and if your tone is bad well… It might stay that way. But depending on the type of music youre wanting to… Which guitars and such seem more like a rockish type…you dont really even need to know how to sing. Sometimes the worse the voice, the better in that type of music. But, if you're wanting tips then you can start from breathing from your diaphram (stomach) [if you have trouble figuring that out, its like how you breathe when you're sleeping] rather than your throat because its putting stress on your vocal chords that way. Drink a lot of water. Get a keyboard of some sort and start playing scales [do re mi fa sol la ti do] its just going from a c to the next octave. Then try to match your voice with the scales and train your voice to pitch. You know, even if your voice sounds bad, the whole point of music is to put your pain or passion behind the sound. So if you do that, it really wont matter what you sound like. =]

Here is the only tip that you will ever need to help you sing better.

Find a singing teacher and go for some offline singing lessons.

Do not use any information about how to sing better that you find on the internet or Y!A. People who know about singing who answer questions on Y!A know that there are no tips that can be given on here to help people sing better so they don't offer them. If you do find tips on here about how to sing better they will be from people who don't know how to sing properly themselves and who don't realise how complicated learning to sing actually is.