I know how to make my final render without noise or grain. But my preview render is always grainy/noisy. Is there a way to fix this?
The quality for the 3d viewport (in rendered view for cycles) is determined by the number of samples for preview.
Or samples for Viewport in 2.8.
You can use more samples, but be aware that it will make the interface less responsive.
Do a search also for Render Border, where you can just render a section of the image