PRE-DOWNLOAD WORD OF WARNING, READ!
Before you continue to the download page you should read the following blog pages, they might answer a lot of your questions before they even form.
Automatic Alpha Masking
User Interface Tour
You may want to visit the Discord Channel too!
Taking a look at the Bugtracker won't hurt either. Maybe the very thing you experience is already known or the suggestion already suggested.
If you're done reading and your head hasn't pop'd like a balloon yet you're free to go. If it did pop, NO GO FOR YOU. Head pop means brain gone, brain is a recommended system requirement.
Proceed to download the Viewer here.
This software is not provided or supported by Linden Lab, the makers of Second Life.