Knowing your English grammar is more important, but you wouldn't be the first English teacher who doesn't know his grammar! It is something you will pick up as you go along. If you know you are going to teach a particular grammar point, look it up in a good grammar book before the class - it's your language, so you shouldn't have too many problems understanding what they mean. If you are surprised with a question in class, you have two options - say "that's too much to discuss now, we'll discuss that next lesson" (which gives you time to look up the answer yourself) or attempt to answer the question by asking your students what they already know - with any luck, you'll come across the answer together.
Don't be embarrassed to say "I don't know, I'll look it up" - that's better than giving the student false information. Students usually understand that you aren't a walking grammar book. If any students do get frustrated, ask them to explain the same point in Spanish - they'll soon shut up!
Some of these Books on Teaching English as a Foreign Language are great for learning the grammar needed to teach English.