Purchase a Spanish/English dictionary and a book on speaking Spanish for beginners. You may also want to purchase a book focused on conversation in Spanish as well as a book about grammar and other language rules.
Enroll in a Spanish class in your school if at all possible. If you are out of school, look into free online Spanish programs such as Study Spanish. (See references.)
Practice on a daily basis, or at least 15 hours a week. Learn sentences as a whole as opposed to learning words separately. Repeat every statement you're learning at least three times before going on to the next.
Learn Spanish with a friend so you both can try to converse and correct each other's mistakes when speaking aloud.
Take your Spanish/English dictionary with you wherever you go. When looking at a billboard or reading a magazine, look up the Spanish translation to get a better grasp of common words.