I got a camera (Canon EOS R5 Mirrorless) for christmas, no instruction manual as my Aunty bought it from a friend. When I turn it on the screen is just black, I think its used for taking pictures but i dont know how.
Canon make all the manuals etc for their cameras available on their web site; here's the R5 manuals from the UK site but you should be able to find the ones for your local area as well.
It is perhaps worth noting here that the R5 is pretty much Canon's top end camera right now, and is to some extent designed for people that know what they're doing with a camera; it won't necessarily have all the features for beginners that an entry-level camera would have. It's also something over £4,000, so you need to say a very big thank you to whoever gave it to you for Christmas.
The Canon EOS R5 is currently the flagship mirrorless from Canon. It can be used for photographing any subject in very high resolution and is capable of recording video in 8K (a few minutes at a time) which is the highest resolution possible for video. It also sports the most capable image-stabilization system which lets you use it in very low light without raising the sensitivity which causes noise.
As pointed out in comments, the camera requires a charge battery inside it to operate. One is always sold with the camera new but if you got it second-hand, make sure you have the batter, it is charged and it is inside the camera. The manual which is available online directly from Canon free of charge indicates all the steps to charge the battery and insert it into the camera. A charger also comes with the camera in the box. If you did not get a charger, buy one, but in the meantime you can charge the R5 via its USB-C port. It is best to charge the battery with a charger since there are fewer chances of knocking the camera or stepping on it accidentally, it does not tie up the camera (assuming another battery is available) and it is usually faster (but I haven't measured with the R5 because I never use USB to charge cameras).
Unless the camera is damaged, you should be seeing something when you turn on the camera. If the screen is black then someone probably switched it to the viewfinder. In the viewfinder (EVF), you will see anything that the camera displays, including the menu. So, if you want to activate the rear screen again, you should use the EVF to invoke the menu and navigate to Settings (Wrench) menu and go down to the Screen/Viewfinder display using a dial or joystick and change it. Auto1 will automatically toggle between the screen being on or off and Auto2 will automatically switch between the screen and EVF. Screen keeps the screen on all the time.
Often such high end cameras are sold without a lens and one is needed to take photos. Even without a lens, it is possible to access the menu system, but you will need to get a lens. Canon makes native lenses for the R5 and those are denoted by
RF, for example the
RF 28-70mm F/2L USM. All those lenses work directly on the R5. Alternatively, if you have Canon EF lenses, you can buy an EF-to-RF adapter (Canon makes several) to connect EF lenses to the R5.