If I could offer you one tip for the future, sunscreen would be it.
On the go
Blot away any excess sunscreen with a paper towel or cloth. Don't rub too hard or you could push grease further into the fabric.
First, blot away excess sunscreen, then rub a good liquid laundry or dishwashing detergent into the stain and wait 5-10 minutes. Then, if the item is white, wash immediately with detergent in the hottest water recommended on the care label.