You can start a conversation from the Contacts menu selecting the name of the contact you want to send the message to and selecting the message icon on the right.
You can start a conversation on the Chats screen in the side menu too. Within this section, you will be able to select the notepad with a pen icon that indicates you can start a new conversation.
After selecting the contact you want to chat with, a chat window will be opened.
After typing your message in, select the paper plane icon on the lower right corner to send the message.
In every single conversation you will have an icon to call the contact and another one to edit the sent messages. This last one will activate just if there are messages in the conversation.
Once you select the edit option, a checkbox will be shown near to every single message of the conversation. From the upper right corner you can erase all the messages, or like the whole conversation selecting Delete all, whereas from the lower left and right corners you can, respectively, copy and eliminate the selected messages.
The active conversations that contain sent or received messages will be shown in the chat's list. You can go to this section pressing the lateral menu's Chat option.