How Do I become a Proofreader/Copyeditor? Training/ School?

BUMP!

Majoring in English is a good start.

I am majoring in Liberal Arts and Communications.

Put heavy emphasis on literature, journalism, and technical writing courses. Along with that, invest in at least two or three of the major Manuals of Style - and master them to the point you are able to discuss, compare, contrast, and critique their assorted points of difference. Read ... read, read, read, read. Read good writers - not just popular writers, but read those writers that professional reviewers and respected literary journals hold to be masters of their craft. Read the time-tested classics, the "Great Books". Proofread, edit and correct schoolmates' essays and treatises. And do your own writing.

A few years of that, and you should be about ready to begin a serious apprenticeship.

Check local area schools of continuing ed. Some may have publishing curricula. NYU has a fairly extensive program in publishing. There may be something similar in your area.

To get an entry-level job, you're going to need strong language skills--grammar, punctuation, spelling, usage. If you're tested, that's what you'll be tested on.

Publishers will not expect you to be an editor or a proofreader fresh out of school. They will expect to train you on the job. However, the more you can start with, the better off you'll be.

The world of publishing is huge, and relatively little of it has to do with fiction. Investigate all the avenues available. In addition to magazines, newpapers (a killer to get into), and books, there's a whole new world of online publishing.

To get an idea of what's out there, I suggest you go to craigslist.com. There's one for New York, Chicago, Boston, and points west (don't remember where exactly). Click on jobs, and then writing/editing. You'll see the enormous array of editing/proofreading jobs in fields that range from dining to medicine.

Timber mentioned style manuals. Most of the publishers I work for use the Chicago Manual of Style. Note that all publishers also have their own in-house style. But CMS will give you a sense of what requires attention when you're editing or proofreading.

I wish you luck.