@Lizzyl: If you're having trouble sending MMS messages, then please try enabling Mobile Data by swiping from the top of the screen to the bottom > Mobile Data, as this is required to send messages that exceed the SMS character limit, or contain images/emojis. If this is the case, you may wish to open the Messages app > Tap the 3 dots in the top right > Settings > More Settings > Multimedia Messages > Set Restrictions, and choose 'Warning'. This will notify you in the event that an SMS has converted to MMS format, in which case Mobile Data will be required to complete the sending process.
Now, go back to Messages > 3 Dots > Settings > More Settings > Text Messages, and make sure that a number is populated in the Message Centre section. If you can't see a number, contact your network directly to request the number, and manually enter it here.