how can first work my MacBook Pro (13-inch, Mid 2012)

User @WheelieNick is correct. The Mac is obsolete, tired and wants to retire. You should let it.

Still, the latest macOS version your MBP can run is 10.15.7 Catalina. If your current OS is older than that, you could upgrade it. But it cannot run macOS 11 or 12 or anything newer.

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The mid 2012 MacBook Pro is obsolete and not compatible with macOS 12 Monterey. The mid 2012 MacBook Pro cannot update further then macOS 10.15 Catalina

The reason that it is slow is also because it is 12 years old. It is 12 years old hardware. In current times of computer development, 12 years is very, very old

Historically, most slowness complaints here regarding that model,—and we have two in the family that are still working—were due to the slow mech hard drive that was the base storage shipped. Both of ours were updated at home with solid-state drives (SSDs); that has kept the computers at least useful. Our SSD upgrades were done in 2018 when that model and its macOS were still fully relevant. I would not do it today.

Adding RAM won't help if you have the base hard drive. Here are launch time changes from my 2012 as I first doubled the RAM, then upgraded to a a proper solid-state drive:

—Base system as shipped:

4GB RAM and slow SATA 3GBps 5400rpm hard drive: Office 2008 and Photoshop Elements 12 took 15-18 seconds to be ready to use.

—First upgrade, double the RAM:

8GB RAM and slow SATA 3GBps 5400rpm hard drive: Office and Photoshop Elements took 15-18 seconds to be ready to use.

—Second upgrade, inexpensive solid-state drive

8GB RAM and fast SATA 6GBps SSD: Office and Photoshop Elements take under 4 seconds to be ready to use.

So did RAM or the SSD help more? My original drive moved data no faster than 80MB/sec per EtreCheck. The aftermarket SSD is doing 500MB/sec, over 6X faster.

As I said, I cannot justify these upgrades today other than as a hobby or learning project. There are fewer and fewer apps and browsers today that will run on 10.15 Catalina.