You should have a Messages icon on your first home screen, touch that. On the bottom of the next screen you should see 3 icons, on for all messages, one for sms/mms (text and picture messages), and one for voicemail maybe (I have visual vm there). At the top the screen you should see a + New message. From there you should get the key board and your address box and composition box. Can you get it from there?
Alternately, there should be an option when viewing your contacts, if you open a contact card, the speech bubble at the bottom will show you their texts, and have "+ New message" at the top. Or, if you just hit the "phone" icon and it brings up your call history, you can press and hold on a number/name and one of the options will be "send text message."