Click the button labelled "Copy Tor Log To Clipboard" that appears in the dialog window when Tor Browser is first connecting to the network. If Tor Browser is already open, click on the Torbutton icon (the small gray onion at the top-right of the screen), then "Tor Network Settings", then "Copy Tor Log To Clipboard". Once you have copied the log, you will be able to paste it into a text editor or email client.