They're phrasal verbs, not idioms. http://www.thefreedictionary.com/ gives you proper examples. Just look them up! As you will see, each phrasal verb has several meanings.

Hello! Telling someone to "get on" would be proposing an action, like "get on the bike" or "get on this website." "Fall out" can mean two different things. it can be used to describe someone "falling out" of something, which is most common. (Example: "Don't fall out of the car.") But it's also slang for people getting into a fight and no longer speaking. Example: My friend and I got into an argument, and we're no longer friends. We had a fall out. "Catch up" is used to tell someone to hurry and get to the same place other people are. Example: You're racing with your friends to the car, and you're not as fast as everyone else. Your friend yells "catch up!" to encourage you to go faster. Another way it's used is to say you have things to tell someone about something that has happened. For example, I have a friend I don't see very often, so when we do get to see each other, we always have to "catch up" on what the other person has been doing. I hope this helps! Send me a message if you need to know more.