1: Just focus on building a good team the gyms and elite4 are hard. you can fill out the dex post game.
2: just finishing the regions and having fun I recommend Unova > Hoenn > Kanto > Sinnoh
3: Sandile is a solid choice you will find it in unova, gengar and lilligant are also good imo. They aren't necessary though you can get by with any mons you desire just note the challenges ahead.
4: The pokedex has a lot of good info like when u learn moves n such. Forums have great info on where to locate items and pokemon also you don't need to worry much about end game stuff, you'll learn as you play. don't be afraid to ask me any questions.
Enjoy The Game!