Decide what part of English grammar you want to learn. Many grammar handbooks are long and dense, and you will have an easier time if you pick a starting point.
Use online resources, such as Libraryspot.com, to learn the basics of grammar. You will need to understand basic grammar terms before you can understand more complicated grammar texts. Internet resources are a good place to get started.
Visit your local book store or library to obtain a grammar handbook or style guide. The MLA Handbook and Chicago Style Guide are two of the most widely recognized grammar books, but you should be able to find a number of other texts that can help you.
Take a writing course or TEFL (Teaching of English as a Foreign Language) course at a local college. Although you may have trouble finding classes that explicitly teach English, writing courses will often cover grammar fundamentals, and TEFL courses will help you learn grammar so you can teach it to others.
Be consistent. You will often be given contradictory information, especially when dealing with complex grammar. Use a single style guide and do not mix contradictory constructions. Even if you are wrong, this will help people understand you.