You don’t know what you don’t know.
Practice makes (as close as one can get to) perfect on the field and off. For a photographer this means the combination of knowing your gear as well as the game you’re shooting.
When it comes to gear, the most difficult transition you may make is going to a longer fixed (or prime) lens. If you’ve been zooming up until that point the learning curve will be steep. If the length is 300mm or more you might need a Sherpa and some rappelling gear.
It takes a lot of exposures to get used to the range. With a long lens the speed of game action is wildly intensified, players are in and out of the frame quickly and following the ball can be a sport unto itself.
Your first few times out I recommend getting as far away from the action as space permits until you can comfortably pan with plenty of lead space. Keep moving back until it’s easy for you to see enough of what’s going on, as you get more comfortable move closer. Eventually you’ll be where you want to be, composing as you like.
The next thing that might take getting used to is a monopod. A monopod is meant to provide stabilization (in this instance) at long focal lengths. Adding a ball head on top can help reduce the rigidity and increase your movement with a minor trade-off on stability.
I’m strong enough to use my 300mm/f2.8 handheld (although prolonged use does cause aches and pains) so I just use a monopod for baseball. With baseball you’re often shooting over a fence which means repeated lifting and standing. For other sports I usually find a comfortable position that involves being on one knee with my supporting elbow on the other knee (or I just sit.)
I like the freedom of not using a monopod, but if you’re going to buy one make sure it’s rated to be able to hold the weight of your heaviest body and lens combination.
As an aside, I recommend you use your lens
fastest largest aperture setting whether you’re in a dark gym or bright sun. The separation of your subject and its background is pure gold.