Simply use the (FIRSTNAME) tag in your messages. For example "Hi, (FIRSTNAME)! Stop in today to get a free soda when you buy a slice of pizza". Also, you can use the Optional1, Optional2 and Optional3 fields with these tags: (OPTIONAL1), (OPTIONAL2), (OPTIONAL3).
Please note: To test this, add yourself to a group and send to just that group. This does not work with the preview, it has to be an actual send.