For the SC I suggest you to get the MGMAT Sentence Correction guide, re-read it a few times until you get a really good grasp of it and practice

I'm going to assume that English isn't your first language. If this is the case and you have the opportunity to, I would recommend attempting to increase your vocabulary first, before you even get any GMAT verbal materials. When you read anything, if you don't know the meaning of a word, write it down and look up the definition. Keep a journal of words and attempt to memorize every word in that journal. After you do this, go through a general grammar book. After you know what things like a subject, an indirect object, a direct object, a predicate, a preposition, etc are, I would recommend going through the Manhattan GMAT guide. I hope this helps.