Kalista's auto attacks are like Draven's axes, except you don't get an indicator on the ground where she's going to land, so just watch her movements for a little bit and you'll get a feel for it.
As far as the Dong goes, you pretty much have your jungler camp him and engage away from his turrets and dodge his grenades and missiles since he has no other escape abilities. Try picking a ranged champ who's AA range is farther than turret range.
As far as laners that you're considering buying, stay away from Ahri and complex mechanical champions. Karma is a good champion, as is Malphite, for your level and forecasted skill level. Champions you can buy in addition are champions like Annie and Ryze. If you're looking for a 4800 IP champion, buy Orianna.
The primary thing you want to focus on right now is improving your game knowledge and decision making and your mechanics. Leave champions like Riven, Yasuo, Zed, Fizz, etc., and come back to them once you've hit max level, understand the game more, and climb to an ELO where champions like this are necessitated. Right now you can simply outplay people with simpler champions and come out victorious. You have to learn your limitations, the enemy champion's limitations, and how to work with that field. Perfect your CS, learn how and when to poke/trade/all-in (but focus 98% on CS), and learn your power spikes and whether or not you have kill potential or can be killed.
The most important thing I can tell you is to not be afraid of a champion matchup. Always go into your lane matchup confident and ready to play your best. Counter picks do not exist until high ELO's in ranked play. There are champions who will have kit advantages, naturally, but learn to play around it.
Example: You play a ranged champion into Zed. He just used his full combo of Shadow, Shuriken, and Shadow Slash, to farm. You can now walk towards him and auto attack him and fire your abilities into him and force him to back off, potentially causing him to lose CS and experience.
Watch Lastshadow9's coaching videos on YouTube. Try to focus all of your attention on the lower ELO ones (silver/gold) and try to learn as much as you can through them and improve your gameplay.
Just because someone is 4 inches taller than you doesn't mean that they can jump higher than you. Remember that and your matches will go just fine.