I assist writers who write jokes (and often write jokes of my own) for TV.
I get materials organized for our morning meeting, which includes choosing the topics we will talk about on that day's show. I gather all the jokes (and help write some), give them to Chelsea, she chooses ones she likes and adds some herself; I keep track and make the cue cards for the show. I get everything ready on set for the taping, we tape that night's show, and then I start to get everything together for the next day's meeting.
I think being creative is most important. Make things. Make videos. Write sketches and film them with friends. Write zines. Write jokes. Write blog posts.
Here's the first step for high school students
Well, if you want to get into comedy, specifically stand-up, start as soon as possible. Go to an open mic. If you are looking for any job in comedy, having stand-up experience will only help you -- especially if you want to be a comedy writer, it will show people you know how to write jokes. Also if you are a frequent performer, it will show people you are a hard worker. Also, write. Always write. Write anything. Read anything. Writing & reading regularly will make you a better writer.
"You're wasting your time focusing on music/comedy/writing, you should be doing something practical." "That stuff will never matter; that stuff will never get you anywhere in life."