Shahabaz, it is not possible to explain here the oscillation principle in detail.
*Just some basic rules: The keyword for an oscillatory circuit is "positive feedback". That means, you have to connect an amplification unit (gain A) and a passive and frequency-dependent network in a closed loop. This loop must have the following properties: The product of A and the damping factor d of the passive network must be unity (A*d=1) at one frequency fo only (the target oscillation frequency). At the same time, the phase shift of this product at fo must be zero.
*This leads to the common rule for a working oscillator (Barkhausen criterion):
Loop gain LG=A*d=1 (that means: real part unity and imaginary part zero).
*To understand the principle, here the most simple example:
The WIEN type oscillator consists of a RC bandpass (damping factor d=1/3 at fo) and an amplifier (opamp with negative feedback) having a again of A=3 - both connected in a closed loop.
Note, the above is a somewhat simplified description of the principle. Perhaps it helps.