To 'get something straight' means to understand something clearly or to understand what is true or to establish the facts of a situation.

1. Let me get this straight - you want to borrow £500 and you can't even roughly say when you'll pay me back? No way!
2. Let me get something straight with you, you can't borrow my things without asking.

1. Make sure you get your facts straight before you submit the article. I don't want to be sued for libel. 
2. Let's get something straight here - I'm the boss, not you, and I will not have my authority undermined!