You may have predictive text input (also known as T9 dictionary) on in your device. Predictive text input helps you to insert 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, the message editor is opened, then select Options > Dictionary > Off or Dictionary off.
To switch on or off the predictive text input, you can also press and hold #.