i took the Digital Image processing subject in college and i am a little confused that how i can find out sampling frequency of a video signal given its bandwidth. For example what will be the sampling frequency of a video signal of bandwidth 18kHz or 15kHz.
It is not clear from your question if your "video signal" is sampled already or if you are trying to find the sampling frequency for an analog video signal.
In either case, assuming your video signal is real, the sampling frequency must be greater than twice the bandwidth of your signal. To allow for anti-alias filtering we typically sample a little higher than that which is a trade-off with the filter design. A reasonable approach is to go 20% higher, but going more relaxes the filtering requirement at the expense of a higher sampling rate.
If your signal was already sampled, and you are actually trying to determine the sampling rate, then you will need some point of reference (since by implication of the question you just have samples but do not know the time between the samples, which is the sampling rate you are trying to determine). In this case you would need to know the frequency content of your signal, and then you could take an FFT to then determine the sampling rate based on mapping your known signal to FFT bin (given the highest bin of the FFT is one sample less than the sampling rate).