Take an idea in your head. Write it out. If you lack ideas to write, join http://writeoff.rogerdodger.me/
for writeoff prompts which will help. Really, the only way to truly start is to throw words into a page. You could make a parody of Harry Potter. Rewrite it, except Harry's gay(?) - and you'd be surprised how drastically different the direction of the story would be when the direction of that awkward teenage sexual tension shifts.
If you lack ideas, it's all about firing your brain into making them. "What if" scenarios are a good place to start - take something accepted as canon, twist it into something else - "What if Derpy was actually a scientist in the Discordian Era who made the alicorns?" - and run with it.
If you have some kind of conflict within yourself, that works too. I started writing my first serious ponyfic because I was irritated at the stupid!Derpy stuff everyone was writing at the time. That led to the above question, and, well, 50k words later I'm still plodding along. But, if you're not invested enough in the fandom or the show to have this, the first idea might work for you.
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