Tap Messages.

Tap the required sender.

You have a number of options when you've read an iMessage.
Choose one of the following options:
Reply to iMessage, go to 3a.
Forward iMessage, go to 3b.
Delete iMessage, go to 3c.

Tap the text input field and key in the required text.

Tap Send.

Tap the edit icon.

Tap the required iMessage to tick it.

When an iMessage is ticked (V), it's selected.

Tap Forward.
Key in the first letters of the required contact.
Matching contacts are displayed.

Tap the required contact.

Tap Send.

Tap the edit icon.

Tap the required iMessage to tick it.

When an iMessage is ticked (V), it's selected.

Tap Delete.

Tap the Home key to return to standby mode.