There's probably the rare story of someone finagling their way into a sound editing gig, first starting out in some other capacity on a film set, then happening upon some kindly senior sound editor who takes them under his wing in the sound booth, teaching the craft like a wise sage to a supplicant.
That's the rare story. It's much more likely that you'll need to go to college and get an associate's or bachelor's degree in videography, or else attend film school. You'll also want to get a part-time job or internship with a network or studio, so you can start dipping your toes into the shallow end of the pool before you get thrown head-first into the deep end.
You might also be surprised at what you can learn while moonlighting as a production assistant. That'll give you a good chance to realize early on if you won't be able to handle the lifestyle—so you can drop out of sound editing school and go back to refrigerator college.
Once you're ready to start freelancing as a sound editor, you'll have to seek out your own work. You might first try scouring online freelancing resources, though eventually you'll probably get your new gigs by word of mouth and networking.