Shiro, if your main motivation for aiming at accreditation is the "fear that the kids are going to be testing [you] and laughing at [you] for [your] lack of knowledge in grammer," you might want to consider that most new teachers with four-year degrees here begin with very young children and move their way up through the age ladder, adding to their Tholean Web of grammar knowledge little by little as they go. You'll have time to prepare, class by class, day by day, and a lot of what you'll need to know will be in the teacher's manuals for the coursebooks you use. Regardless, you should never have to feel backed into a corner by a grammar question. Chances are that if a student asks you for a rule regarding structure that you can't furnish right away, then that sought-after rule is likely fraught with exceptions. Even what you think is the most basic 'rule' can make a liar out of you. You know to correct "Dog's hungry" with "The dog's hungry" because that's just what people say when there's a specific, commonly understood subject... hey, or do they? Hmmm, people do say "Dog's hungry, gotta feed it, be right back," and "Car won't start." These statements are entirely acceptable in spoken and literary discourse. If a student asks you an incisive question, tell her what you've heard, read and/or used. If she hypothesizes usage that's 'grammatically correct' but sounds like it came through a wormhole from the Delta Quadrant, just say it could be right but you've never heard it before. You'll have more than enough time to check out real instances of use from a language database (or get input from the thousands and thousands of other English teachers online) and come back next class with enough information to make you look majorly impressive.
I surmise a bigger challenge than grammar for you coming out of the gate might be that of classroom management. The debate rages over how much and what kind of control and structure you should impose on your students and your classes, but my feeling is that, at least in the beginning, a lot beats not enough. Now, I'm not the edumacated type (as is obvious, everything I needed to know about life I learned from Star Trek), so I can't tell you how much or how little you can learn about managing students and classes by going to school. Perhaps some of the others here in this forum have some experience and/or knowledge about this?
Also, if you get ESL/EFL education, you'll become acquainted immediately with issues relating to methodology, phonetics, etc., which may or may not save you trial-and-error time on the job. A lot of people have gone right from an undergrad program into teaching, and of these many have never felt the need to return to school. Others have saved up their quatloos for accreditation by teaching, and I dare say these folks know much more of what they're talking about than those going into such courses cold.
Keep your options open, but if you're serious about getting certification for language teaching, you should probably get at least a Master's Degree. That way (should you ever want) you could even get a full-time post at a college. I forget exactly on which Star Date those Universal Language Translators are due to be developed, but I think it's safe to say it won't be in our lifetimes, so it seems likely that, at least somewhere in the world, you'll always have English teaching to fall back on. You could do worse than get credentials saying you can do something for which there'll always be a demand...