The audio mixer has mute buttons and volume sliders for you to adjust in case viewers report problems. Try to have your microphone going into the yellow, and your game likely just below the yellow. Do not go into the red – this wont sound good to viewers.
Any media source will have its own mixer, even those that don't play any audio (such as your video backgrounds). You can click the small cog next to a source to hide it.
When you add your microphone as a source, it's likely to not sound the best it could. Luckily, OBS has some easy filters you can add to tweak this. In any scene where you have your microphone source, right click on the source and select "Filters"
The filters I tend to use are:
If you are using a webcam, you may find the webcam's frame rate doesn't look very smooth, or that your image is washed out or otherwise incorrect. Open your webcam source's properties and check the options.