Some general tips, courtesy of my very own experience:
1.) Read a lot.
Preferably, read a lot of comics, especially of the type you'd like to write. Reading gag-a-day comics is great fun, and very educational when it comes to learning comedic timing - but if you want to write an adventure comic, you're going to want to read adventure comics to learn pacing and characterisation and framing of shots, etc..
Also, don't just read comics. Read everything. Read novels, read non-fiction books, read articles, read poetry. You never know where you might learn something, or where you might find inspiration. Maybe reading a book about the history of the crusades won't immediately give you ideas - but one day, you might find yourself considering doing a comic about a crusader and then voilá! You'll already know the basics!
2.) Practise makes perfect.
So you want to write a comic. Great! Now go write some comics. Like, now.
You can read all you want, and learn all the theory you want - but you have to get in the water to actually learn how to swim, you know?
Don't start of with a big, sprawling epic. Start off with small stuff! Do one-shot comics, or even just single scenes. Start small and build your way up to something bigger. I might be working on a huge comic now, but I've done a bunch of smaller, shorter comics first - everything from 5-10 page stuff that are exercises in specific stuff (like fight-scenes, or explorations of settings) to longer one-shots of 30-50 pages, telling the full arc of a short story.
And it's really helped. When you know how to handle the building blocks of a story - scenes, dialogue, character-development, establishing shots, action sequences, where and how to place speechbubbles, etc., - it's much, much easier to put together a big story.
3.) Set yourself deadlines.
You don't have to stress out and finish stuff in record-time - but setting yourself some kind of goal in terms of how long it should take to finish something means you will have to actually work on it. If you don't have some sort of self-imposed deadline, it's very easy to keep putting it off until later. And then you never finish.
... aaand that's been my experience learning how to write. Don't worry about getting it right the first time - failing is one of the best ways to learn.
