I like to think of zazen or meditation as just sitting mindfully. You don't do anything for a while. You slow down. No, more than slow down. You stop.
You plop your butt down on a pillow or meditation cushion, you face a blank wall, you cross your legs in whatever comfortable fashion you can do, you place your hands in a similarly comfortable position, either palms up or down and on your knees or your right hand in your lap and your left hand on top of your right hand with your thumbs lightly touching, you wiggle and adjust until you get comfortable, you get still, you sit still, you keep your eyes open, and you breathe.
You notice your breathe. Maybe you count your breath up to ten and then start over again. Maybe you just keep breathing and noticing your breath whenever you can. Over and over again.
Your mind will wander. You'll think of things: the past, the present, the future; the mundane, the ordinary, the extraordinary, the profound; the things you need to pick up at the grocery store, the ways you'd like to improve yourself.
And when you realize you've thought all this stuff, just return to your breath. Set aside the thought right now and return to your breath.
Maybe you'll start thinking about that thing again. You'll finish that grocery list in your head because that's what you plan to do right after you meditate. That's okay.
Sometimes you'll spend the entire time thinking. Sometimes you'll return to your breath often. Sometimes your mind will feel clear and spacious. You'll think eureka! This is it! This is meditation! This is enlightenment! This is the blank mind the Zen masters speak of! This is what I want when I meditate! But this thinking is an illusion, because meditation isn't about any of this.
Meditation is about just sitting. Just sitting there and no matter what happens in your mind, sitting for a bit. That is what is important. That is what just sitting is about. You took the time to stop and sit. Whether it's 10 minutes or 15 or 25 or 35 minutes. Whatever you did. Fantastic! You carved out time from your day and you just sat.