There isn't much to the video settings in OBS, so this part should be relatively simple.
For Base Canvas, select the resolution that is in the settings of you game. Most times, this is the resolution of your PC monitors but it is best to check.
The Output (Scaled) Resolution is what you want your stream and/or recording to be. It can be the same as the monitor/game resolution or you can change it.
If you have the option to change the Downscale Filter, it is suggested that it is set to Lanczos (sharpening scaling, 36 samples).
If this option causes FPS problems, you can drop it to the middle option. Most times FPS issues are not a problem. If you do not have the option to change the Downscale Filter, your PC mostlikely does not require downscaling.
The Common FPS Values sets the FPS of your stream or recording. The most common FPS values are 30 and 60 but it is dependent on your internet.
Be sure to click the "Apply" button at the bottom to ensure that your settings have saved