Your turtle is probably a red eared slider with a red stripe on its face or a yellow bellied slider that is very similar but without the red stripe the turtle egg is probably not fertilized but if you want put it in a well ventilated container and keep it 75-85 for about 2 months and see what happens there should be more than one egg at leased 4 if not the turtle can be retaining eggs and you should take her to a vet
Buy a inkubater there cheap they are like 50$ to 100$ there cheap
Well if your turtle has an egg the best thing to do with it is don't touch it, but keep an eye on it and when it hatches take the turtle to the place the parents are and let the parents teach it stuff!
Don't touch or move it!!! It will hatch soon!!!just calm down!! Ok?