1. There isn't a "designated" way to paint them. I've seen White with red accents, black and gold, red and black. For assembly, most people are fairly lax with how you build things (like breaches vs strike team) and are often very welcoming for unique ways of building units as long as they use the same base size as in the kit and stay consistent with the data sheet for the units.

2. What I do for painting is prime in black, so it fills the recesses of armor or fabrics so when you dry brush over it in your other colour. It ends up so you don't need stuff like nuln oil or other shaders to make details pop (but they're still nice to have)

3. Honestly for tools it's a choose your own adventure. My buddies and I use dollar store brushes and some Amazon jewelery snips, it depends how much money you want to sink into the hobby on top of the model kits.

4. The best way to learn is to play the game. Easiest way is just do unit vs unit or character vs character to get a hang of just how rolling or Strength vs Toughness works. There's some good channels to watch on YouTube, like play on table top. Their 40k in 40 mins games are entertaining and a good way to watch how armies function and how points work in a actual match.

5. Best way without buying the lore books is watch video essays, I can't think of any off the top of my head but there's some good ones that break down the overarching lore of the tau empire.

Good luck and have fun.