What will help early gastroenteritis to stop it from getting worse?

Time. Most gastroenteritis is caused by a virus. Time will help most as your bodies immune system will be your defense. Drink lots of fluids but it will need to run its course. If it is lasting longer than expected call your doctor.
Fluids/ rest. Start w clear liquid like water, slightly flattened soda that is clear, or sports hydration drinks. Sip, sip, sip. Work up to solids w bland foods like bananas, rice, applesauce, toast, gelatin, or chicken. Avoid alcohol, nicotine, caffeine, dairy & high fat foods. Rest.