When you get a stain on something, deal with it immediately. If you let it dry, it’s permanent.
Your cloths will wind up much cleaner if you separate them not just by color, also by fabric type and water temperature.
If you aren’t weighting your laundry, you should fill your washer about three-quarters of the way to the top of the drum.
Before you add your clothes, add your detergent, allowing it to dissolve in the water fully before adding your clothes.