What happens if one day aliens want to talk to us. How can we talk to them? Always I wonder about that if some day alien wants to talk to us, then how can we talk to them? They didn't know any of the language on the Earth.

What will be the basic building blocks for it?
Some basics:

Whether we listen for or send signals into space, something to keep in mind is that space is full of EM radiation of all wavelengths, from very long wavelength background radiation, to a range of microwave, infrared, visible and UV light from stars to gamma-rays from accretion disks and collapsing stars.   Any signals we send and perhaps any signals we could receive would be like raindrops in an ocean.   There's an obvious range to listen called the hydrogen line, which is the emptiest region of EM radiation in space.

Even so, across light years, signals would still be faint, so the first signal that's sent needs to be an easily recognizable signal and math is very good for that.   Pi in binary or a series of repeating prime numbers wouldn't happen naturally, so signals like that are a great way of saying "We are here".

Once the first signals of prime numbers or other math are exchanged, and we see what direction the signal comes from and we know where to listen and where to send, then the communication can begin, presumably using more traditional language.