Ask your teacher about getting a tutor to help you spell in Spanish. Your teacher might work with you one on one to improve your spelling, or they might help you find a tutor who can sit down with you each day or each week to work on spelling. There are several methods a tutor will use, and if you can't get a tutor, use these methods on your own.
Make flashcards of Spanish words spelled correctly, and then show yourself the English side of the card. Write down what you think the Spanish word is, and then check your work. Continue to practice with a set of flashcards until you have mastered it and then make a new one. Write letters in Spanish and run them through the Spanish spell check on your word processing program. Write down the words you spell wrong and make flashcards or practice those that you've spelled wrong.
Use a dictionary online or a paper dictionary. You can find them online at sites like SpanishDict.com and other sites. Or, buy one at a bookstore, second-hand or from your school. Look up words that you do not know how to spell as you are writing them. Each time you need to look up a word, write that word on a separate piece of paper. Later, write that word correctly five times. If you do this each time you have to write something, you'll learn how to spell the words more quickly.