Every photographer needs to understand the capabilities and limitations of their mix of equipment. Here, let's consider looking at only lenses and camera bodies. The combinations of lenses with differing focal lengths and cameras of different sensor sizes can be mind boggling. Nobody can have all possible lenses for all possible camera bodies, and have an immediate grasp of how all these affect each other and the pictures they produce. It simply isn't practical, nor is it necessary.
New photographers sometimes find it difficult to comprehend all the implications. Visualizing how one lens behaves on cameras with different sensor sizes is something that needs to be learned and understood. It affects the very outcome of the picture you are taking.
Fortunately, to help visualize the differences between lenses on different cameras, Canon has produced the Field of View Comparator:
This is pretty nice. In the left-hand column, in the Camera A box, select your preferred camera, perhaps the one you own and use the most. Then, in the Camera B box, select a camera you'd like to use for comparison. Typically, this would be one that uses a different kind of sensor chip than the one in Camera A. In the picture above I have selected a crop-sensor camera for Camera A, my Canon 60D, and a full-frame camera for Camera B, a Canon 5D Mk III.
Next, move the Focal Length slider to the value you'd like to compare. Here, I have set it to 50 mm. You can see, as the two cameras you selected are looking at the exact same scene, how the differing focal lengths and different type of sensors change the scope of the picture you will get. With the example above, the 50 mm lens on a full-frame camera sees a much wider view that the one produced by the same lens on a crop-sensor camera. In fact, the crop-sensor produces a picture that is 1.6 times closer to the subject. (See the crop factor values indicated in the lower right-hand of the picture.)
So, if you are using a camera that has a sensor smaller than a full-frame camera, you can use the crop factor to calculate the effective focal length a particular lens will have on that camera. In this case, the 50 mm lens will produce a picture on my Canon 60D with an effective focal length of 80 mm. (50 mm x 1.6 crop factor) This is good to remember, because focal lengths for lenses are designated by their full-frame values. My Canon EF-S 18-200 mm lens for my crop-sensor Canon 60D in reality has an effective range of 29-320 mm. It is a longer lens, effectively, than it would be if it could be mounted on a full-frame camera.
Play around with various values and camera selections. It will help give you a better idea of the capabilities of your current gear, and perhaps give you a direction and priority for purchasing new equipment.