just practise, practise, and practise. when spelling out words say them out loud and once you've spelt them see if they look right. and don't type or text in "text language" it doesn't help, it just encourages laziness in spelling!
Some of the good things which can help you improve in your spelling are reading more books, playing word games like Scrabble and Boggle. Word puzzle is also another way to improve on your spelling among other things.
It would help immensely if you had a friend or relative to go over your work with you a night or two a week. Many times spelling errors come from using the wrong words, for example: its or it's, or their or there, or hear or here. Do you use spell check on the computer? Even a spell check program will not help in those instances because it can't distinguish which spelling is correct for your sentence. Reading is a great help to work on your spelling, whether its a book or a newspaper -- even song lyrics -- and either look up any words you don't know the meaning of in a dictionary or ask someone for a definition. This will also build your vocabulary. Mainly, find someone who will help you. Look around; you may even have a retired neighbor who would work with you. Good luck!
Get a list of SAT words from the internet. Study them then ask your Mom, Dad, siblings and other relatives to quiz you. If you have an iPad or iPhone you can also download a spelling app. There are other spelling apps out there- google spelling apps.