Hi and welcome to the forum:
As my respected colleague, Keith, explained you need the EF-RF Canon adapter. Avoid third-party ones, they have issues.
The 75-300 is a slow lens, so you want to keep a reasonably open aperture because it does not have image stabilization to help stop camera movement when hand-holding it. For that reason, how you hold the camera is critical. If you are used to a cell phone you have been holding the phone in front of you to look at the LCD, but a camera is much heavier and with longer focal lenses, any tiny movement by you will be significantly magnified and result in blurry images, so holding the camera with the viewfinder to your eye is critical, to help create a tripod effect between the camera supported at your eye, and an arm tucked into your torso at each side.
Work with the diagrams below to cradle the camera with your left hand, heel under the camera body and fingers under and around the base of the lens. The left hand should take the weight of the camera and lens, leaving the right hand to manipulate the controls.
Initially, you can use the P and Auto+ modes, but take control away from the camera, and use creative modes like Av and Tv, or even M, you need to learn about the 'holy trinity' of exposure control: Shutter Speed, ISO and Aperture. The combination of all three will get you a correctly exposed image, but each one has a profound impact on the creative side of the resultant image. So, you should get some training under your belt.
The second thing you need to master is the actual control system of the camera itself - the better you know it, the more it does not get in your way and the more creative you can be. To that end, I recommend downloading the PDF copy of the Advanced User Guide from: c015.pdf (start.canon).
To get you started on the principles of photography, I recommend the following:
Learn Photography - Simple, Practical - Free Photography Course 1/10 (youtube.com)
and the following series of guides from Canon Canon EOS 101: Photo and Videography Basics | Canon U.S.A., Inc.
To help you get to know your controls, watch this video:
Canon EOS R100 User's Guide (youtube.com)
Canon R100 Tutorial - Complete Beginner Guide (youtube.com)
Getting to grips with photography with a dedicated camera will take time and patience, but you will be rewarded with an ability to really have creative control and great images.
