Learn the English alphabet. The website "Learn English Basics" has a chart of the entire English alphabet plus audio files for pronunciation. It also has the alphabet broken down into vowels and consonants and a chart of the phonetic alphabet. The same site also features resources about capitalization and common symbols and punctuation.
Once you have done this, or if you already know the alphabet, begin building you vocabulary. The British Council has some excellent resources for vocabulary building on its site. The BC has vocabulary organized by theme, such as sports and food, and you can then test and practice your knowledge through quizzes and games, such as an online version of Hangman, a vocabulary race, and the Learning Ladder, an online adaptation of the board game Snakes (or Chutes) and Ladders.
Once you've begun building up a good stock of vocabulary, you're going to need to learn some grammar in order to use it. The BBC's Skillwise page has several grammar resources suitable for adults learning English. There are fact sheets, worksheets, and quizzes for you to learn and practice adverbs, verb tenses, pronouns, apostrophes, subject-verb agreement, adjectives, instructions, putting together simple sentences, joining sentences, and how to use commas. Skillwise also has areas for you to practice spelling, increase your vocabulary, and practice very important listening skills.
Develop your reading skills. The Skillwise page also has a section for you to learn English reading elements, such as skimming, summarizing, scanning, and recognizing different types of text. If you just want to practice reading and comprehension, the same site also has a section filled with English stories sent in by students in the UK.
Don't neglect your writing. The more you write, the better you will both learn and remember vocabulary and grammar. Skillwise has a writing section, organized into getting help with format and structure, proofreading, and paragraphs. The website "Rong Chang" also has a great number of ways to practice English writing online. There are activities to help you make yes/no sentences or "wh-" questions, practice tenses, unscramble sentences, use the passive voice, noun clauses, and adjective clauses, and more.
Practice your listening. There are literally hundreds of podcasts developed for teaching and practicing English as a Second Language. The Rong Chang site has an extensive list of ESL podcasts. Download the podcasts or just listen online.