I heard that a lot of stuff on your desktop significantly reduces your mac speed.
It can, depending on the Mac and the number of folders. The CPU and GPU need to constantly redraw the desktop when you open a window. The more stuff 'stored' on the desktop the more it has to redraw every time you open or move a window. Macs with lots of memory, a fast CPU, a good GPU, and solid state storage can make this a non-issue. Lesser-hung Macs might see a slowdown.
How you speed up your Mac depends on exactly what Mac you have. Replacing a spinning platter hard drive (HDD) with a solid state drive (SSD) will definitely speed up a Mac. But that's not possible with some, unless you boot from an external.
Give your storage drive (Hdd or SSD) about 20% free space. Keep your OS clean as possible. Some people reinstall a fresh copy of the OS once a year. Make sure all your apps are compatible for your OS. Make sure your hardware supports any OS you want to use. Some OSs can tax older Mac gear. Run DiskWarrior on your OS from time to time. (A pricey app, but it's been well worth my money for years!)
Restart your Mac holding the Shift key until you see a log in screen. Log in, and when in,
immediately restart. What this does is start up in Safe Boot and run First Aid. Much functionality is turned off to do this, so that's why you immediately restart, to get back to normal.
Again, what Mac you have determines what you can do to speed it up, apart from general OS care.