There are a few things you'll need to start shooting video with a gimbal. First is choosing a gimbal, based on the camera you have. Gimbals come in a variety of sizes, and each has a size and weight limit for the camera it can support. There are minor differences between gimbals, but the majority of them have motors on three axis—the pan, tilt, and roll axis—and they are meant to be held using two hands, though some have one-handed modes as well.
Some gimbals are made for pocket or action cams, some are made for lightweight mirrorless cameras or small camcorders, and others are heavy duty rigs that can handle a DSLR or cinema camera with a big lens. There are also tiny gimbals that have built-in cameras for extremely compact shooting.
Many gimbals require a stand for balancing, and you'll need to take the stand with you on any shoot, in case the gimbal goes out of balance. Conveniently, some gimbals are designed to be used without a stand, so you can balance on any tabletop, or even by connecting the gimbal to the top of a tripod.
Accessories can be helpful, but they can also add extra bulk and complexity to the setup. For example, an external monitor can help you see what you're shooting better than using the camera's default LCD, but the added weight and setup can get in the way of a pleasant, reliable shooting experience. That also goes for a remote or toggle accessory, which can help you pan and tilt the camera in place, but is often more complicated than simply using the gimbal's default pan and tilt "follow mode."
But the one accessory that is always recommended—not just for gimbals, but for any camera support such as a tripod or monopod—is a quick release system. In this case, being able to click your camera in and out of the gimbal also means you don't have to rebalance it every time you secure the camera to the gimbal.
Most importantly, you want to choose a gimbal whose load capacity can handle your camera and lens. If you have several lenses, start with your widest lens, weigh it with a quick release and anything else you may have on (like a microphone, for example) and then find a gimbal that works.