First of all. DO NOT BUY any more gear. A lot of amateur photographers just buy gear thinking it will improve their skills or improve photos they take. But trust me all you'll end up with is unused gear sitting on your shelves and of course lighter wallet.
Limit your self to the gear you have and learn how to use that gear.
Once you have mastered all of your gear then figure out how that gear is limiting you. By this time, you will gain enough experience and knowledge to make better, educated and well-informed decision. You will know what gear you need based on your photographic needs.
As for the original question(s): Based on your gear, I would suggest:
I am looking to improve my basketball, soccer, football, and baseball photos
Take your camera, set it to highest ISO possible. Set your aperture to its widest possible setting (opening). Ex. f/2.8 is wider and will allow more light in than f/4. Set your camera to A- Aperture priority mode. Then go to your local high school or university and take pictures; practice. Think of getting some kind of support, monopod, tripod to limit the movement you impose on the camera. Sports photography requires anticipation to get the shot you want. Search Google for panning, sports photography.
ISO - Think of ISO as workers collecting water with buckets. More workers you have, more water you'll collect. So increasing ISO will 'collect' more light and give you ability to shoot in low-light, fast moving, etc scenarios. Just be careful, ISO has direct relationship with noise. So increasing it will sometimes create unacceptable image.
and also scenery photos:
Again based on your equipment: I would suggest:
You're pretty much all set to shot this kind of photography with 18-55 lens. When it comes to scenic photography think of: mood, composition and leading lines. Also know that any lens is at its sharpest at the mid aperture range. Usually around f/8. For scenic photography i would: get a tripod (a must IMO), set you camera to LOWEST ISO setting, set your camera to A-aperture priority mode and set the aperture to f/8. Shoot away.
Again this is just quick-to-get-you-started advice. When it comes to photography there are so many things not mentioned in this short answer. But don't be alarmed and don't worry too much about getting 'perfect exposure' but instead experiment and just take photos.
One book I would recommend though is Understanding Exposure, search for it on Amazon.
Hope that helps.