How does hydrogen emit radiations of diffrent wavelengths and still is called to have line spectrum? Things have mixed up completely in my mind.
Firstly Hydrogen is an element so it produces a line spectrum. But the famous Bohr model for the Hydrogen spectrum, confuses me about how different radiations can be produced due to the excitation and relaxation of its one and only electron. In the same way, how elements can have only one line spectrum while they have a lot of electrons ... it is like I can't get the meaning of line spectrum. 
I know what is the emission of radiation of specific wavelength, but how does it work with elements that have a lot of electrons? Which specific electron that excited and then relaxed, would emit radiation that we can call the line spectrum of the body? 
P.S. I am still student ... I am just looking forward to learning stuff...
 A: As you said each line in the spectrum represents a photon of a specific wavelength. The photon is simply the form of energy released when an electron transitions from a higher energy level to a lower one. 
As the precise energies of the energy levels are discrete (as opposed to bring energy bands, so to speak) the possible differences between them are also discrete. 
As the wavelengths of photons emitted depends on the difference in energy between the two energy levels concerned, the wavelengths of photons are also discrete. Hence the sharp, distinct lines on the spectrum. 
An electron can occupy lots of different energy levels; if you give the electron some more energy, it will jump to a higher energy level. But the exact energy level it jumps to depends on how much energy you give it. This is one of the reasons for the various wavelengths being emitted - the electron falling from different energy levels to the ground state (lowest energy level). 
But as well as simply falling straight to the ground state, the electron can fall to, say, the next energy level and then to the ground state. Or it could fall to the next, and then to the next, and then to the ground state. As you can see, there are lots of different possible transitions. This is why there are different wavelengths seen on the spectrum for hydrogen.  
I hope this was clear and some help. 
A: Okay, so even if you have one electron, there are lots of different energy states it can be in. (If you're still at the most basic level, think that there are a lot of allowed "orbits" of different energies.)
Photons can be absorbed or emitted if they have an energy equal to a difference between two energy states.
If an atom has a lot of electrons, then that means that there are a lot more energies to consider! There is still one "spectrum" but a photon could cause a very complicated change in the state of the electrons in the atom.
However, electrons, like other particles, "want" to be in the lowest-energy state. And electrons cannot simultaneously "share" a state (although it turns out electrons have an intrinsic property called "spin" which can take on 2 values, so one "orbital state" is equal to two "electron states" in general). So if the atom is cold enough, most of these electrons will occupy the "lowest" energy states, and transitions will be dominated by the "boundary" energies. That is why you see distinctive band patterns even when there are lots of electrons: only a few of these electrons really do the hard work of absorbing light with the states available to them; most of them are just happy where they are.
A: So, one by one:

how different radiations can be produced due to the excitation and relaxation of its one and only electron?

The excitation of the electron (by photoelectric absorption of a photon, or by Compton scattering) is to different energetic levels. In stars, at the huge temperatures the electron of H can be raised to any high energy level of the H atom.

In the same way, how elements can have only one line spectrum while they have a lot of electrons.

No, it's not one-line spectrum, it is that some elements have a certain line typical to them, between two specific levels.
