There is no pressure on the shower, how can I repair it?

Remove the shower head.
Inside the connection between the head and the pipe is a small mesh screen. It apparently has clogged. Simply knock out all the granules.
The shower head itself may also be the problem. The spray openings may be clogged. You can either try and clean them, or spring for a new one. They aren’t that expensive.

I don’t know how it works where you live, but here in Spain that means one of two things. 1— your pipes are full of cal and need acid to clear them out, 2— the water tank on the roof is almost empty.

try to unscrew the shower head, maybe crud from water line. could be calcium (hardness) buildup. just soak head with holes down in a shallow dish in vinegar for a couple hours, should dissolve any calcium buildup

When you go to put the shower head on, you’re going to need plumbers tape to wrap the threads where you screw the shower head back on to stop it from leaking.

If your renting than call the maintaince men. It could be anything from the showerhead to the water tank to a leaky pipe somewhere.
Best way to tell if its the showerhead is look at the pressure below in the faucet. If its great than its the showerhead. If pressure in the faucet or in other faucets around the house is also poor than the problem is in the delivery from the water tank.

The shower head is completely clean, Its less three months old.

I closed the water from the complete apartment, then opened the shower and closed every other faucet. Reopened water to create pressure on the shower and it something got removed in the pipes because although I dont have the usual pressure is much better.

You most likely have debris in the shower head. (95%)Use two pairs of pliers. Use rags to protect the finish on the head and arm. Grab the pipe that comes out of the wall with one. Use the other to unscrew the shower head from the arm. Turn on the shower with the head removed to flush the shower supply line. If it comes out real strong, the clog is in the head. If you have three handles, the middle one switches from tub to shower, and the flow is still weak, the clog is in the shower diverter valve.(middle handle). If it’s in the diverter, call landlord or a plumber.
When you reinstall the shower head, wrap teflon thread tape on the threads of the arm before you screw on the shower head.
The clog came from the city water, it doesn’t matter how new or old the head is.

some of the newer heads have a water saver restricting part inside that tapers the orifice down so less water sprays out. The smaller an opening is, the easier it will eventually clog. Mine was cone shaped so I used a razor knife to cut all of it off except the ring part that was the gasket that was in the connection. Now it comes out with the full force that it takes to rinse off instead of waiting all morning to clean up.