On Phone X and later, you can see the battery percentage in Control Center. Just swipe down from the top-right corner of your display.
On iPhone 8 and earlier, you can see the battery percentage in the status bar. Go to Settings > Battery and turn on Battery Percentage. When you have Low Power Mode on, the battery percentage will always show in the status bar. Battery Percentage isn't available on iPod touch.
Learn how to charge the battery on your iPhone, iPad, or iPod touch.