Use :set spell to turn on spell-checking.

:help spell will give you all the details.

Here are some excerpts:

To search for the next misspelled word:

]s           Move to next misspelled word after the cursor.  
             A count before the command can be used to repeat.
             'wrapscan' applies.

[s           Like "]s" but search backwards, find the misspelled
             word before the cursor.  

Finding suggestions for bad words:

z=           For the word under/after the cursor, suggest correctly
             spelled words.

To add words to your own word list:

zg           Add word under the cursor as a good word

To remove words from your own word list:

zw           Like "zg" but mark the word as a wrong (bad) word.

I don’t put :set spell in my .vimrc because when I’m coding there are just

too many variable names in my comments that get flagged.

If there is a certain file type you want checked use an auto-command in your .vimrc.

Or just turn it on manually when you need it.

Use :set nospell to turn off spell-checking.

Use :set spelllang=en to change the language used in spell-checking.

Also see :help set spelllang for information on changing your dictionary to

include other regions, languages, or word sets.