I don't claim to know all. But this is what's worked for me:
Composing

I want to talk about "composing" as a seperate subject. If you want to be a true composer, got to school. Learn traditional music theory, instrumentation. Conducting is a big plus. You'll probably have to learn multiple instruments (we're not talking bass and lead here, folks...).

If some introduces themselves as a "composer" to me, then they're a minor god in my eyes.

However, there are legions of musicians who also lay claim to the title, but its not the same thing. Modern synths can do the most fantastic things! And that's cool! Because that's how the bulk of music for TV is done. You don't have to be a keys player, a drummer... you just have to know your synth really well.

To write effective music, you'll have to know your market cold. For instance, Atlanta is home to Turner broadcasting. Perhaps you can get into writing show "bumpers" (you know, those little diddies they play before they break to commercial from the movie of the week). But you better know what their style is... and you learn THAT by watching tons of Turner. Detroit is cars; so, car commercials. Washington, DC has the Discovery Channel networks, National Geographic TV, PBS, and political ad agencies.

If you're looking to record music to picture (i.e. video, usually), you'll need to get some technical chops, too. You will be REQUIRED to work with SMPTE Timecode (SMPTE= "Society of Motion picture and Television Engineers"... you know, Zappa's "Baby Snakes"?). And it a continuing point of amazement to me that so many composers just don't get what timecode is all about. There's nothing more frustrating to post production mixers to get a music package with the score "locked" (in sync) to timecode and have the cues come in at the wrong length. And THEN have the stones to ask "what you do to the music?" My point here is never, NEVER say that to a post mixer... would you yell at a priest or a nun? Neither a post production mixer. Because this is the person that's gonna save your ass with the film's producer by editing the music to correct length. The key here is communication. Not sure if its going to clock-out right? Talk with the post mixer. He'll be glad to help.

That about wraps what I have to say on writing music for TV. Any thoughts and questions are most welcome!

Best Regards

Karl