Ideally, every time you go out to shoot footage, whether it be for leisure or for a specific project, you carried along a tripod to keep your camera steady. But we all know that occasionally that's not possible—either you forgot it, couldn't bring it, it got broken, or you simply don't have one—which is why it's important to know how to stabilize your camera when you find yourself without. There are loads of things you can do, like making some DIY stabilizers or using stuff from home to absorb any shock, but the first thing you should know how to do is how to properly hold your camera. This video from Adorama shows you some techniques for doing just that.
That's super basic—it's the kind of thing that might proceed learning how to turn your camera on, so let's talk about some other not so obvious things you can do to stabilize your camera. (Some of these things you've probably already done!)
These tips from Filmora are pretty helpful, especially because they utilize things you can usually find lying around in your house, car, or even in your pocket. You can attach a string to your camera to create a makeshift tripod using your feet as a base, set your camera on some sturdy objects, like books, tables, etc., attach a rubber band to your belt loop and attach the other side to your camera, or even cut a hole in a shopping bag and put your camera inside.
If you want a good DIY project, you could build a shoulder rig out of $70 worth of equipment, a DIY Steadicam with off the shelf parts for $100, or a handheld stabilizer out of LEGOs. These won't necessarily help you if you've forgotten your tripod (you probably won't find a bucket of LEGOs just lying around somewhere), but they are pretty nifty alternatives.
And of course, if you're shooting with a GoPro, you can always use this important technique: hold the damn thing against your face. GENIUS!