Well it depends on the media but let's say it is a blog. First just write about something, anything that catches your fancy. Leave it for a day or two at least then reread it and if you seem that it is something you wish to publish pick a random blog site or medium and create a username not attached to you and just post it there, and if the several blog posts after that seem to be under the similar subject or idea your blog has got an overarching topic or theme.

Then just continue to post there when you want to say something and over time you will get wuote a collection and probably a few readers as well.

Now this is how it is done from a standpoint of a person that just wants to write and doesn't want to profit in any way from it rather than to have a place where they can manifest their thoughts, so if you are in it for something more I don't have any useful advice.

One of my first post, in retrospect, where so shallow and badly written but I am fond of them because it I'd because of that person that I am able to now much better express my thoughts, so don't fall in the trap of perfection, it is that imperfection that makes it unique. All perfection is the same, as such it has no room to grow, and that kinda misses the point for me.

Good luck, be honest yo yourself about your creation but dont be judgmental, see it as a learning process that will just happen to last your whole life :)