enter a new contact. Make it somebody you can have right there with you as you do it and figure it out. Use a simple name for them right now, something like 1aatest, so it shows up first on your contact list. Enter the whole area code and phone number.
Save the contact and go home
tap the message app
tap the "To:" field
enter the name (1aaatest). it will probably need you to enter the whole thing, without helping you
enter your message
hit send.
Now, next time you open your messaging app "1aaatest" will be right there onscreen as a choice. If you want to message somebody else, you can enter choose them if they're in your contact list, or enter their phone number.
An easier way is to open your contacts. Notice that when you tap a contact, along the right margin next to their phone number is a little icon that represents the messaging app, it's to the right of the telephone handset icon. Tap that messaging icon and you're on your way.
As you text people they will show up in the message list when you open the app, and also as "recent" in the messaging app, and "favorites" in your contacts.