Hey there! You have cast quite a wide net with those questions
#1 - You will NEED the codex for any army you play.
#2 - You will NEED to review the Rulebook
FAQ, and
FAQ for any codex you play as well, which can be found here:
https://www.warhammer-community.com/faqs/?orderby=post_created&order=desc#warhammer-40000
#3 - In order to play competitively you will NEED to adjust the points costs listed in the codex with the new ones found in Chapter Approved 2019. 99 times out of 100 though, if you use a program like Battlescribe, you'll be fine.
#4 - T'au don't have a lot of competitive builds at the moment, but their MAIN competitive build is extremely strong! If you have 3 Riptides and a bunch of Shield Drones, you're looking to be in a good spot.
#5 - Chaos is one of the most "in flux" armies in the game, and the competitive lists for them can end up with lots of extremely varied units. However, here's the units that tend to pop up the most frequently:
- Ahriman on a Disc
- Daemon Princes w/Wings
- Sorcerers
- Lord Discordants
- Chaos Knights (Knight Despoiler variety)
- Obliterators
- Dark Apostle
Here's some Chaos Units that have an on-again/off-again habit of popping up, so they can be good at times (like Possessed right now), and then totally disappear at others:
- Possessed
- Khorne Berzerkers
- Forgeworld Dreadnaughts
- either of the Primarchs (Mortarion and/or Magnus)
- Plaguebearers
- Havocs
Thankfully, both Chaos and T'au present to you some great options at not being a horde list!
#6 - Most competitive army in the game is currently the loyalist Space Marines. Their codex is just... wow. It's a cut above the rest. So much so that
GW has released a kind of emergency
FAQ to tone them down a bit. Casually, you'll find Space Marines are quite potent, but manageable, as people don't go nuts, especially with how the emergency
FAQ is toning them down. But COMPETITIVELY you'll likely still find that they sit at the top. Also very competitive are:
- Chaos Soup (mix of Chaos Space Marines & Chaos Knights, also some Daemons)
- T'au (one or two specific builds)
- Craftworld Eldar (now called Asuryani)
- Necrons (one specific build)
- Tyranids (anything that slams Genestealers flying up fast, what I call "Nid Rocket")
- Orks w/Da Jump & Da Souped Up Shokka
You can keep up with who is winning where and with what here for the tippy-top stuff:
https://www.40kstats.com/
Good luck!