Welcome to Android Central! First, make sure mobile data is on (3G/4G), because MMS (i.e., picture texts) doesn't use voice signal or wi-fi. Then compose a text, and tap the attachment button (either a paperclip icon or a photo icon), and select the photo to attach. Videos are difficult to send, because the maximum filesize you can attach is 1.2 MB for a message--most videos are much bigger than this. If you want to send a video, upload it to your Google Photos or YouTube, then send the link privately to the recipient.