Just as you know when to write:
"I meet or meat my friends."
"I want to write/right a book."
And which is the correct: "by, bye, buy" or "their, there, they're" English speakers learn each word individually and so do Greeks. Only in Greek there are fewer exceptions.
Your best friend for spelling is the Drop Down Hints to see which is the right translation and then learn it. Each exercise contains only one new word and they are repeated many times so you can learn them.
Here are some other links you should be using:
You'll find advice on the alphabet. On where to find resources and much more.
And for any questions jsut ask us.
If you want to start Greek look at this page where there are questions and answers to help you.