If you send SMS/MMS to someone, you will have to bear the cellular charges applicable unless you have subscribed to a free text-messaging plan. You can use the iMessage app to send texts, photos, or videos from iPad tablets or Mac PCs to other iOS devices or iMacs over Wi-Fi or cellular data networks. To iMessage someone, you need cellular data or Wi-Fi, so both incur charges in a sense. However, sending an iMessage will help you add more texts than messaging someone with character limits, as the feature will route text messages via Apple’s servers.
Tap on the Settings icon denoted by turning gears and scroll down to the left-side menu until you locate the Messages on your device. Tap on the Messages menu to access iMessage in Settings. It shall be noted iMessage should be turned On by default on iPad tablets. However, if the slider next to iMessage is set to Off in Settings, then tap on the slider to enable it. Note that the iOS will prompt you to log in with your Apple ID to avail this feature on the device.
You will need to configure how you can be reached on iMessage, so tap on the Send & Receive button just below the Send Read Receipts menu. If you have an iPhone tied to the same Apple ID, you can see two phone numbers listed there. If you have several iPhones logged into the same address, you may see several phone numbers listed on Send & Receive menu. The same goes for e-mail addresses attached to the Apple account, so choose either your contact or the email ID to configure how you can be reached on iMessage.
If you want to attach a separate email ID other than your personal email ID to send or receive iMessage, then tap on Add Another Email for that. The iMessage app will help you to send emails from contacts in that email account or text someone from your phonebook. Choose the settings in Send & Receive accordingly; you can configure it later as well, when you start using iMessage on your iPad.
The encrypted iMessages would appear in blue text bubbles and the standard messages or multimedia messages will appear in green text bubbles on your iPad tablets so you can differentiate both easily.