try soaking with Dawn liguid dish soap...don't rinse it out before throwing it in the washer
Shampoo works too. Use an old toothbrush to pre treat, then launder.
Lava bar soap
Susan,. Try a combination of a little water, and hydrogen peroxide, half and half,. Brush with a stiff brush, not too hard. If that don't work, try baby powder. Rub it over the stain to make it less noticeable. I'd take it to the cleaners, that's there expertise, getting out stains. Aloha!
Fels Naptha Bar soap scrubbed into stains will usually remove them.
if its ring around the collar on the neck try wetting the area and apply some baking soda..leave wet for a couple of hours and then soap it up and wash. If the stain is something particular on the front of the collar from food or what have you try applying some salt to the wet collar and brush out with your old toothbrush you keep for cleaning jobs (provided the brush is clean). Other stains from wine etc,. look up 'stain removal' on google for the correct remedy. All the best! Do not bleach if the shirt is denim, coloured or nylon..some white nylon turns yellow with bleaching.