hello guys I'm 20 years old, I'm music freak. i love sing songs and rap of Linkin Park and especially of Mike Shinoda and Eminem. My friends told me that I can sing better than common people, but my problem is I can't sing in a high voice; I don't have much knowledge of singing. Please guide me friends.
I have two answers, but first a caveat: I'm a singer, and I firmly believe that, as a singer, a solid technique should let you sing in any style — but rap isn't really my thing (like, I don't even recognize the name "Mike Shinoda"), so my advice is going to be about basic, healthy vocal technique in general (especially for men, which I'm guessing you are one of from your question, though the same general principles hold if you're a female Sam).
The absolute best thing you can do is find a voice teacher. There is really no substitute for an experienced singer who can listen and give you feedback in real time. The voice is oddly more complicated than instruments. On the one hand, everyone can sing, and singers with no training can get a lot farther on intuitive talent and genetic luck than, say, untrained trombonists. On the other hand, though, there are no buttons to press on your voice — the entire mechanism of the instrument is entirely invisible and uses parts of your body, like the soft palate and intercostal muscles and base of the tongue, that you mighty never otherwise have been aware of — so it's much more difficult than for most instruments to read someone's general advice and make it happen.
You don't need to spend a fortune or make a big commitment to get some lessons. If you're a student, you might be able to ask a friend (or a stranger) who's a student in the music department if they'd be willing to work with you a few times — I've done this for friends, and aside from helping them I think it was a really good experience for me. You could also join an amateur choir (good directors will be trying to help their singers improve their technique, though obviously there isn't much individual attention, which would be the most useful). The best, of course, would be an experienced voice teacher, and as long as you're up front about it, no teacher that works with beginners would object to your taking a few lessons without committing to years of study.
All of that said, I'm going to try to give you some general advice.
I'm sure you already know that some people naturally lower voices and some higher ones. However, whatever biological equipment you may have, almost every male singer has trouble accessing the higher part of their voice. (Plenty of female singers do, too, but it's less comically dramatic for them as a rule.) It is a constant goal of of voice training to become better at crossing the passagio (a.k.a. "bridge," "gap," or "break") between the high ("head") voice and low ("chest") voice more smoothly and elegantly, so that the registers blend seamlessly together. The goal here is to sound like the same person is singing the high notes that is singing the low notes. (In some styles, like yodeling, that contrast can be used as a "special effect," but even in that case, I argue that it works much better if you choose to add more contrast in than if you don't have the technical chops to be able to make a choice.) That is a big goal for men and women, but it's a more obvious when male singers aren't doing it well, and many men have the additional step of learning to use their upper register in the first place, before they even start worrying about joining it up with the lower register.
The absolute fundamental for singing is good, supportive breathing. Books can be written just about that, but breathing wasn't your question, so I'm just going to reiterate a few basics: when you sing, you should stand in a loose, comfortable posture, with your feet shoulder-width apart, your shoulders rolled back (to lift and expand the chest — human spines don't go "straight up," but that's the idea), and a level head (i.e. neither crunching your chin down nor tilting up at the ceiling). You want your tongue relaxed on the floor of your mouth and (especially for this) your soft palate raised.
Your breath itself should be athletic, but more in a Michael Phelps way than an NFL linebacker way. The primary action should be your lower abdominal muscles moving your gut down and out, which lets the diaphragm lower and fill your lungs with air. You'll also feel a very subtle expansion in the muscles between your ribs and in the small of your back, but the prevailing movement is down and out. If this sounds new to you (or even if it doesn't), the best way to check up on breathing is to put one hand at or just below your navel and the other in the small of you back. When you breathe, you should be able to feel a little expansion in your back, the hand in front should move really significantly, and your shoulders and upper body should be basically still: none of that dramatic-looking heaving we see when people are hyperventilating.
With the breath support in place, the un-sexy solution to getting to know your upper register is lots and lots of vocal exercises. I think a really useful place to start is with ones that don't seem like singing: big, dramatic (but gentle!) sighs from high to low, lip trills going up and down in a sort of continuous circle, etc. The more detailed work will come in singing, essentially, up and down the scale. I would do
do re mi fa sol fa mi re doas the exercise to start, and move up a half-step from pattern to pattern. You want to take this very slowly, and pay attention to what the note sounds like: the point isn't just to check off a box that you've sung it, or you won't make progress. You want every note to be very resonant and clear and even, and you should adjust each note to refine it if it's sub-par. Pick a sound for the exercise that you think is easy for you to sing really resonantly and beautifully. I personally favor "see" and "say". When you find a problem spot, you'll want to check in on your breathing, try to "place" the note (I describe it as feeling the sound between my eyebrows), and experiment with different ways of shaping the vowel/different amounts of openness in your mouth to get the resonance you're looking for.
The most important thing is that nothing should ever feel forced or tense or strained or pushed, and certainly not painful. You want to be relaxed and loose, but still strong and supported.
I hope this is helpful! The best thing (aside from finding a teacher) is to keep singing lots and lots and enjoying it. If there's anything I said that you'd like more explanation about, just ask — I'm really happy to talk about singing, I just don't want the answer to be too frighteningly long.