Goto to join.me website.
Click on the Share button.
Depending on which browser you use, you may be able to select Run or it may download the program.
If it downloads then double click the program to run it.
If your anti-virus comes up with a warning click OK or ignore (again depending on the AV you use).
Once the program is running a 9 digit number will appear at the top middle of your screen.
Give this number to the person you want to share a screen with.