I would agree with Anbesol about shooting in Aperture Priority Mode. I would however disagree on shooting a small aperture as mentioned. The difference between a nice action shot and an attention grabbing shot often is in the background or rather lack of clear background. You want to concentrate on the action and have it in focus while the background and foreground are out of focus, (nice bokeh) to make the action pop.

To do this, set your lens wide open and your ISO to 100 and you mode to AV and you focus mode in continues focus. If the D90 will let you set you auto focus to another button than the shutter release then do so. Canon allows for autofocus to be set to a button on the back of the body right under your thumb.

Check your shutter speed. It needs to be at least 1/320. 1/500 would be ideal. Sports like baseball, football don't need shutter speeds faster than 1/500 to freeze the action. If you are at 1/320 shoot away. If it is below 1/320 then crank you ISO up to 200 and check again. Keep cranking the ISO up to get the minimum shutter speed you need.

Once you feel comfortable doing this then it is time to consider switching to full manual. In full manual you will constantly be checking the meter to make sure your setting are right but it is worth it. This takes more concentration but the total control is nice in difficult shooting situations.