From the Inbox, tap the New message icon (lower-right).
From the 'New conversation' screen, enter the contact's name or a 10-digit mobile number in the 'Recipient' field. Alternatively, tap the Contact icon (upper-right)to select one or more recipients from your contacts list then tap Done.
As the number or contact is entered, a matching contact may appear and can be selected at any time.
To add an attachment:
Tap the Plus icon + (left of the 'Enter message' field).