Maladaptive Daydreaming: where wild minds come to rest
Just start, it's all you have to do realy.
If you want to write good you just have to write a lot. It is like practise and you will get more experience the more you write :)
My advice would also be to get back into writing first, by writing idk what. And only then, after some time whene you're back in the game, start writing that one story that you realy wanted to write. Because otherwise you might look back on it afterwards and see that you don't like your writing from whene you just started, and you might abandon the story. Start as quick as possible, but take some time to start serously so the awesomeness of the story fits the awesomeness of your writing skills ;)
Of course you don't actually have to bother about writing good. Just starting to write whatever you want at whatever pace you want is realy fun, and if you only want to share it with some friends or close relatives, than just go ahead and write that one awesome story ;)