It depends on your device, but on most phones you’ll find it in keyboard settings. On Android, go to Settings > System > Languages & input > On-screen keyboard > your keyboard (like Gboard) > Text correction > Auto-correction, and turn it on; on iPhone, it’s Settings > General > Keyboard > Auto-Correction. I usually keep it on since it fixes quick typos, but it can get annoying with slang, so you might tweak it a bit after turning it on.