Sounds like this could be a challenge for you. Most of the non-tech world just goes to Apple store and buys a new iMac, so I commend you for taking on this challenge. @Ausdauersportler has given you good advice as to building an OpenCore SD card to boot your new card with, though I suspect your bigger challenge may be to flash the VBIOS on your new card, and on installing it in the iMac. Proper prep and review is required to do that successfully, especially for a non-tech person. Torx drivers, thermal paste and K5 PRO or thermal pads are amongst the items you should have on hand. I find pre-flashing the new card before installation using a CH341a and clip is the best way to go, but if you don't want to buy one (they are cheap), or the card you select has a BIOS that can't be clipped, then there are lots of alternative ways. I think you will find that the alternatives exist because everyone has different supplies on hand, and so different ways to proceed. It can be overwhelming, but pick one that seems to fit your situation and go with it. There is a wealth of information in post #1, so read and follow its links.I just said what i need from my machine. It wasn't a questions. So i need guidance how to make brightness control and boot screen to work step by step. The bad thing is that is my first iMac. I was an windows user. So ... no idea how to make a bootable USB stick for OS or anything like that.
Since you have decided to stay with High Sierra, you have certainly lessened the technical post-install steps, but the flashing and replacement of the card certainly requires some technical aptitude, or at least a desire to learn. Good luck.
One last thought, if you plan to stick with High Sierra, then you don't really need one of the video cards that support metal. Why not pick up a working Apple card off eBay, and save yourself the trouble of OpenCore, and the flashing of a non-Apple card. You will still have to go through the disassembly and swap process, but an Apple card will support native brightness, boot menu and such. You may even find that all that is needed is new thermal paste on your CPU and GPU, and what you already have will work again.
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