What is did that query show you? What is the recovery model and what was the log reuse wait description? Some of what you do depends on what those tell you. Check those first and let us know what it says.

For the other questions, backing up the log allows space in the file to be reused. That's what they call truncating the log. It's not the same as shrinking the log - so you don't want to confuse the two.
You can take differential backups when you're in simple recovery but they aren't likely to affect the log at all.

Sue