Hi,it seems your phone is in T9 mode which is the default text mode for sony ericssson phones,
please follow these steps;

Go to the messages on your menu,

From there go to create new messages,

After which you go to the writing options,

when you open these option,you will find the T9 mode,

Click on it,the on and off options appear,

click on off,and your T9 formatt ic deactivated instantly.

Take care