Select a message 

Click on the message. 

Select a group of contiguous messages 

Press-and-hold Shift. Click on the first message in the group, then click on the last message in the group.

Select multiple messages 

Press-and-hold Ctrl. Click on the messages that you want to select.

Select all messages in a folder 

Choose Edit -> Select All.

Invert the selection of messages 

Choose Edit -> Invert Selection. The messages that are currently selected become deselected, and the messages that are not selected become selected.