There's a button at the bottom of the screen when you post, in between the preview and rest button. Use it. Simple.
There are other ways, such as using a Mircosoft Word. It helps you with spellings and grammar. Or you could use www.spellcheck.net an online spell checkerer.
dont say you havent been told.
You can also get Mozilla Firefox, it is a free download and it is wonderful. it works a lot like Microsoft Word, if you mis spell a word, a red line will go underneath that word, and you can right click it for words spelled like what you wrote.