Most sports are challenging to photograph for the same reason: The action is fast-paced. Athletes are constantly running and jumping and throwing. Autofocus is key -- sports are constantly in motion, and throwing away a shot because it's out of focus is a real heartbreaker.
Shooting football or baseball or soccer presents another wrinkle: The fields are really, really big. The drama's in the close-up, and getting a close-up of a football player may require one heck of a lens. Ever seen a DSLR attached to a telephoto lens that's about two feet long? That kind of lens.
You don't have to spend thousands on a lens, but buying the best one you can afford will define the quality and range of photos you can take on the field. A good telephoto lens will give you the range to zoom in on faraway subjects. Just as importantly, it gives you control over background blur with strong depth of field. And picture quality is always paramount: Lenses like the CanonEF 100-400mmf4.5-5.6 LISUSM deliver a massive focal length and a sharp picture.
A 300mm or 400mm lens provides ample focal length for zooming in on the action, but not all sports need to be shot at extreme distances. Indoor games such as basketball call for smaller lenses with larger apertures to handle harsh lighting. Covering different sports requires a variety of equipment, which leads us to our next tip: Always having the right equipment on hand.