Did you know that how you cut your meats directly affects its tenderness? The reason is because muscle fibers run in a specific direction, and the longer those fibers in the piece of meat you're eating the less tender it will be. This alignment of the muscle fibers is called the "grain" of that piece of meat, and the goal is to cut across the grain to shorten those fibers as much as possible and thus increase the tenderness of the steak.
In the image above, you can clearly see the direction the fibers are aligned on a piece of steak. This is easier to see in some steaks than others, and is also more visible before they are cooked. Once you've identified the direction of the grain, you'll want to slice perpendicular to that grain, whether you're slicing the steak on your own plate or ahead of time if you are cutting a larger steak to share with others.
So how do you know if you sliced your steak properly? Refer to the image above. On the left you see a steak with no visible muscle fibers, while on the right you see the long fibers running parallel to each other. In this case, the steak on the left will taste much more tender, while the steak on the right and its longer fibers will taste more chewy.
This will take some trial and error at first, so don't be afraid to slice your steak starting on one end and compare your results to the image above. If it looks good, go ahead and eat! If you see those long fibers, rotate the steak 90 degrees and try again. And remember, the same applies to all cuts of meat, so make sure when you slice, you go against the grain!