I just downloaded the Ausacity app and i am trying to remove backround noises from an audio (it’s an audio of people speaking but there are backround noises that need to be removed).
How can I remove the unwanted noises?
It depends on the nature and severity of the noise. Pros still record in soundproof studios with good equipment and good mic placement, etc., because there is only so-much you can do with software. 
You can try the [u]Noise Reduction Effect[/u] first. You feed it a sample of noise-only and it tries to separate the signal from the noise. But if the noise is bad, "the cure can be worse than the disease. It works best with a very-slight and constant background noise… It works best when you don’t really need it.
If you have a hum or whistle at one frequency you can use a notch filter to kill that frequency. (You can use Analyze → Plot Spectrum to help find the frequency.)
With voice recordings, you can use the Graphic Equalizer (or a high-pass filter) to remove the low (bass) frequencies that are below the normal voice range, since everything in that frequency range is noise.