Hi! So, the key if you want to teach "grade school" is: which grades and what subjects?
For secondary (grade 7-12) you need a specific subject focus - to be a "highly qualified teacher" in your subject you need typically about 30 undergrad credits plus maybe 9 credits in a related subject, so with your history degree you could probably get certified as a middle or high school history or social studies teacher without going back to school. You would get paid better with a masters, of course, and you're going to need to find some way to get a "planned course of study" specifically in how to teach. I researched this last year and ended up finding an MA in Teaching (MAT) degree. You don't study any particular content area, you learn how to be a master teacher. You then put it to practice in your content area. I'm about half way through and LOVIN' IT.
For elementary (grade PK-6), you just need a bachelors and a "planned course of study". Again, an MAT might be a good option and there are tons of schools offering them around the country.
The other option for you might be teachforamerica - check out their website. It is kind of like the peace corps, except the teachers get trained in a teaching boot camp then get sent into the toughest schools on a 2 year contract. At the end you get to leave or sign up to become a full time teacher. No better training that some of the toughest schools in the country - AND you're doing GOOD WORK at the same time. I looked into it, but turns out I'm too OLD (!).
Anyway, the thing to do initially is check the requirements for certification in your state... the state department of ed will probably have something on their website explaining, and go with what you need for the subject area you want to teach. Right now most states are struggling for math and science teachers, and possibly english as a second language teachers.
Email me james (at) patsalides (dot) com if you'd like to discuss more. I'm half way through making this transition and would be happy to help... ;-)