You may have predictive text input (also known as T9 dictionary) on in your phone. Predictive text input helps you to type the messages more quickly than using traditional text input. When using the predictive text input, press each key once for a single letter.
To switch off the predictive text input, select Menu > Messages > Create message, then in the message editor select Options > Prediction > Off.
To switch on or off the predictive text input, you can also press and hold #, then select Prediction off.