You can start researching here --
What kind of television show are you thinking of making? That may play a huge part in what you are trying to do.
No matter what kind of TV you're going into, you'll likely need to start by understanding the parts of a story. During High School (don't know what age you are), start by taking a lot of creative writing courses. As you read stories, start to break it down into elements: Beginning, middle, and end of stories. See how characters develop and how the setting plays a part in the story. That's the first step.
Once you're off to college, start to do the following things, as much as you can:
--Study improv. Many colleges have improv groups. If yours doesn't, there's a good chance there is an improv group near your area.
--Learn stand up comedy (many comedy clubs offer these classes). This will teach you a lot about how comedy is actually structured.
--Watch a TV show you like. Write up profiles of how the characters fit together. How do they interact? What needs do they need? (Characters are based primarily off need. In Finding Nemo, the need of the father was to find Nemo...this built his character. For the black and white fish in the fish tank, his need was to get out and get to the ocean. That defined his character. The blue fish that went with Nemo's dad - his need was to have just one friend. That drove his character). Once you understand the characters needs and dynamic with each other, try writing an episode. It is your first try, so it doesn't have to be perfect. Watch out for "gagging," where you insert a funny line just to be funny. This is the first mistake most people make. Make humor comes from the characters...not the funny lines.
A book that might help is "The Comic Toolbox: How to be Funny Even if You're Not." You can find it here:
The author has written for a few TV shows (Murphy Brown, Head of the Class, Married...With Children). He has a section on sketch comedy and one on TV comedy. He also leads you through many exercises throughout the book that help prepare you for comedy, comedy writing, and performing comedy.
Realize a lot of this is just luck. I have a friend who did a TV show pilot episode with great comedians who were famous enough to be a "no brainer" for the networks. The show went well in the test markets. Everything seemed set to go. Suddenly, the network just canceled the show with no real reason. It's just sometimes a matter of luck.
My experience has been about comedy because that's what I perform and know best. There may be slightly different ideas for other types of TV shows, but I'm sure a lot of that will follow the same.
For majors in college:
Good luck and hope to enjoy something of yours on the tube sometime! You might also want to keep up on my own comedy group's web site, which will likely soon start posting an improv blog if enough people say they have time to post to it regularly:
Boston University, just one example, has an excellent program.