How do i get a flat stomach?

Most people don't have one especially if they're female as your muscles and the positioning of the uterus makes it very difficult. That small protrusion below the button is pretty much where your muscle and reproductive organs are (buried deep) and tilting forward a little causes this. It's more common than not and a good sign. Emaciate yourself and you'll still have it so what's the point in trying? You're not overweight at all

try crunchs xxx

It could be your posture. If you have an anterior pelvic tilt it makes your stomach stick out.

If you lie on your back on the floor and you have a big arch under your back then that's what it is. You should just have a small gap that you can slide your flat hand into.

If that's the issue some useful exercises are: to actively push your back to the floor using your core muscles, to push from long sitting to reverse support, to practice crawling on hands and knees keeping your back flat, in standing to put one hand on your bum and one on your lower abdomen and press them closer together while thinking about growing tall through the crown of your head.