You want to write something. You've tried flying by the seat of your pants before, and it works in its own way, but you keep hearing people tell you to PLAN and PLOT ahead of time. But it's hard to know where to start if you've never done it before!
Some of this you've probably done already. Some of it you may have thought about doing but didn't do it or didn't know how to go about doing it... so here is my really simplistic view on it all. I figure we all need somewhere to start, and this can work from short stories to novels to epic trilogies. Heck, you could plan out a whole trilogy, plot-wise, before writing it! Wouldn't you be awesome with your interwoven plot themes and all that? Yeah you would!!!
OK, so here we go!
First off, figure out who your main character is, and anyone else who is important to them. If you don't know, off the bat, who the "bad guy"(antagonist) is, that's OK! It will come to you.
Write it down. Names. How do they look? How old? Interests? Dislikes? Pet peeves? How do they dress? How do they act? What kinds of people are they friends with?
Do this for every person you create. If you come up with a new person in the middle of your book, when it's a natural stopping point, take a minute to write them one of these character sketches too.
Where does your story take place? If it is a fictional world entirely, are there any rules you need to tell us about? Magic, (how does it work?) creatures (tell yourself about them!), space-time continuum stuff needs to be explained, at least to yourself. YOU need to know the rules so the audience knows there ARE rules. This is easier to do if you make them up ahead of time! Really!
Once you're done with your character and world sketches, get started on your outline. This doesn't have to be a big drawn out thing up front. Start small.
Figure out the basics: Beginning, middle, end. Write down the big points. Then expand on it.
What happens in the beginning, maybe 3 points you want to hit on.
What happens in the middle, maybe 3 points you want to hit on.
What happens in the end, maybe 3 more points.
Then go in and add transition points. What happens between beginning and middle to GET you to the middle? Middle to end?
It doesn't have to be super long or drawn out, but basics. Start slow for your outline and grow it from there.
You are allowed to change your mind, of course. And you're allowed to bend the rules or the order, etc, but this will help you along if you get stuck while you're writing. You get stuck? You look at your outline. What comes next? Write toward it.
Just try it! See how it works for you!