You can sit on a variety of things. It doesn’t have to be a meditation cushion… Chairs, pillows, etc all can work. The key is sitting in an upright, comfortable posture that is stable and allows for relatively comfortable sitting (ideally without moving) for 20 minutes or more. Whatever you sit on, it should allow your back to be straight, your knees to be slightly lower than your hips and your hands to be comfortably resting somewhere on your thighs. (hint: if sitting in a chair or a couch, make sure you sit up straight, don’t lean back, and you probably will want an additional cushion under your butt to make sure your hips are higher than your knees).
Here is a nice little article from Meditation For Dummies with some basic posture instruction: