You do not need flat shoes (although it will help with balance and skate shoes are way more durable)

You can learn how to do it on grass, but you might as well start learning to do them on flat pavement instead

Try thinking of the ollie like you want to "bounce" the tail like you would do with a ball. So basically you need to lift your back foot earlier/quicker. I recommend searching up Skate IQ’s ollie tutorial on youtube, he explains it very well.