The TextEdit application contains a handy feature called Transformations (Edit ➪ Transformations from the menu) that allows you to convert any text that you select into uppercase, lowercase, or use it to capitalize the first letter of every word.
This came in handy for me just recently. Someone sent me a document by e-mail that contained no capitalization whatsoever. Since I had to edit this document, I needed to fix the lack of capitalization. Using the Transformations feature in TextEdit, I held down the ⌘ key and selected the first word of every sentence (holding down the Command key allows you to select non-contiguous words). Then, I chose Edit ➪ Transformations ➪ Capitalize from the menu. This capitalized the first word of every sentence. Done!