First try just washing it in hot water! THis should get it out. If not, keep reading.
1. 1 cup of white vinegar 2. 2 cups water
Step 1
In a big enough tub, mix that cup of white vinegar with 2 cups of warm water (not boiling).
Step 2
Soak your sweat-stained white fabric in the tub for 30 minutes.
Step 3
Wash with hot water.