If you want a starting point, look at your own writing and correct it. This way, you focus on your own problems with grammar, instead of trying to deal with it in a general sense.

Well, I would first tell you to think about what kind of learner you are. Do you learn things better when you can read them, and take notes about them? Do you learn best when you listen to someone tell you how to do something? Do you learn best when you get to work with what you are learning? Do you like classrooms and group learning, or do you prefer just you and a teacher? If you know the answers to those questions, then you can choose the best type of lesson for you. If you think you can learn well by yourself, or just you and a teacher, and you prefer less formal lessons, then this website might be a really good place to learn grammar. Grammar is a hard subject-- it's kind of like the math of language. It has right and wrong answers, and it is pretty technical. But, there are a lot of really experienced teachers on this website, and one of us may be able to help you with grammar. I, personally, have many years of experience studying grammar and teaching it. If you think an online lesson is a good fit for you, then go looking for a teacher here! :) Best of luck.