It's pretty easy, since the phone has a keyboard. To text someone, you click on the envelope icon, click the menu button, scroll down and click Compose SMS Text. You will then either type in a phone number/email address or pick someone from your contacts.
After you type your text, you click the menu button again and type S to send.
Of the many phones I have owned I can honestly say that the Curve is the easiest one I have ever text on. The phone is the easiest and most intuitive device I've had, and I've had regular phones, smart phones, and PocketPC phones.
I've been kicking myself for not migrating to the BlackBerry sooner.