Sending a text message on the iPhone is easy.
From your home screen (aka SpringBoard), press the Messages icon.
From the window that pops up, press the New Message icon in the top left.
Type the name of your contact you want to send to until they show up, and then press them. If the contact is not already in your iPhone, simply type their number in the To field.
Type your message, and press Send!
That’s it… you’ve sent a text message!