FAQ:
(Based on what I've asked, got answered and seen in the previous thread, will improve with time)

Q: How do I know when a stat is maxed out?
A: The game will tell you.

Q: What's with that bar on the right?
A: That bar indicates how much EV you can put overall in your poke, once it's filled it'll be fully trained and can't get any more.

Q: I fucked up, I want to start again to focus on Atk, what do.
A: Hit the blank bag until you get a Reset Bag.

Q: What's that green portion inside the yellow portion?
A: That's the base stats of your pokemon, without any changes.
They indicate where your pokemon excels at, for example, you can expect Machamp to have that green stuff pointing a lot towards Attack. You can use that to improve said stat with EV.

Q: Any tips?
A: IMO is best to improve the stats your pokemon excels at instead of making it a jack of all trades (master at none). Don't try to increase Blastoise's Speed when it's supposed to be tanky as hell.

Grab a pokemon you don't care about and use it to unlock the Lvl 3 Training Regime, then use those to train your really beloved pokemon.