Start with basic areas, such as parts of speech and types of sentences (statements, questions, commands). You can find clear, easy definitions and examples on websites such as Scientific Psychic (see link in Resources below.)
Go on to other topics such as adjectives and adverbs, conditionals and conjunctions, participles and prepositions. Many websites, including LEO Network, introduce a wide range of grammar help at no charge (see ink in Resources)
Get into advanced grammar, with topics such as "advanced unreal conditional" and "modal verbs." You can find free tutorials on these subjects by visiting a website, such as EnglishPage.com (see link in Resources).
Take an interactive tutorial--English Grammar 101 is one example. You often pay for these courses, but you receive feedback on your progress through pre- and post-tests or a tutor's help.