iMessage is on by default. You can see this under settings/messages. Assuming this is on, then iMessage is an automatic process. If you are texting to another iOS5 device, the text bubbles will be blue when iMessage is active. If you're texting to a non-iOS5 device then the messages automatically go via SMS (green bubble). The advantage to iMessage is that it doesn't count against any SMS plan you have with your carrier. Of course if you're unlimited, this wouldn't be an issue.