1. Let's break this down into a list.
1. Be sure that you understand the purpose of a thesis statement and how to write one. A good thesis statement will directly convey the focus of your essay (see question/answer below on thesis statements).
2. Understand the process of writing, as well as the requirements of a successful essay (see links below).
3. If you're struggling with grammar, trust me, you're not alone. The English language is confusing—especially when writing. Even if you've been speaking English since childhood, I'm sure there are words that you find confusing and nearly impossible to spell. I'm an English teacher, and I still wrestle with the language! Add grammar rules to the mix and it can be a confusing mess!
Writing in grammatically correct form can only occur when you understand the rules. If your high school English teacher dropped the ball by not driving the rules of grammar into your head, or if you couldn't learn because you were too focused on friends and fun, then it's understandable why you struggle with grammar today. That's ok—it's not too late to learn. In fact, you have to learn if you want to write effective papers.
I'm going to recommend a site that will give you a 'crash course' in grammar and writing. Please review the site in full. Bookmark the page and take notes. This is a very helpful site: http://www.grammarly.com/handbook
4. Now, as horrible as this sounds, when writing your essay, stop after every sentence you've written and do two things: read the sentence aloud and check for grammar errors—every. single. sentence. Reading your sentences aloud will help you 'hear' any poor grammar.