There’s probably the rare story of someone finagling their way into a sound editing gig by starting out in some other capacity on a film set and happening across some kindly senior sound editor who takes them under their wing and teaches them the craft, but much more likely than not 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’ll learn as much or more about the job moonlighting as a production assistant as you will in your classes (and hopefully make some invaluable connections), but both are pretty necessary. This will also 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, you’ll have to seek out your own work. At first by scouring online freelancing resources, and eventually more by word of mouth and networking. Hopefully you don’t foul things up on your first couple of jobs and have industry people badmouthing you, because that is not the type of word of mouth you want.