Fold marks in the fabric can be removed by using an iron or clothes steamer to help loosen the fabric fibers and drop the wrinkles or, alternatively, if you are in a rush, you could consider a homemade linen spray to help dampen the fabric and carefully stretch the crease out of the garment.
You can prevent deep fold lines in clothes by hanging items that crease easily in your closet, rolling garments, or folding clothes more carefully folding to help line clothes up in your dressers rather than stacking them and pressing the folds in under the weight of other garments.