Begin by typing up everything you want to say and then continue to re-read and eliminate words until only the very essence of the thought remains. Edit freely or have someone with good editing skills review your work. Intersperse bold words when you want to emphasize something or draw attention to a particular point. Remember that bold face type is only effective if it is used prudently. Keep text boxes to a minimum and don't put them all over the place because it will force readers to jump all over the page.
White space is often better than words.
Text should appear near any corresponding graphics. This way, readers can quickly glimpse at an image and decide if they want to read the accompanying caption. Don't make your visitor scroll and scroll and scroll to continue reading. Try to limit long text to just one screen at a time. Granted, when a reader is truly interested, every word will be absorbed but the majority of site visitors want to glean the necessary information and move on.