I'll try and list the basic things.
Types: Each Pokemon has one or two types. They give the Pokemon weaknesses and resistances. If you use a move of the same type, the power goes up.
Items: Items can be given to a Pokemon to gain various effects. You can give an oran berry to heal health, but not a potion. You can, however, use a potion in battle.
Catching Pokemon: You deal damage to a wild Pokemon, so it gets into the red / yellow zone. Then, you throw a pokeball. It may or may not catch it. The more damage you deal, the easier it gets.
Battling trainers: The Pokemon they send out you can't catch. You want to deal as much damage as you can so their Pokemon faint. However, you don't want your Pokemon to faint.
Exp: As you battle, faint, and catch Pokemon, the Pokemon you use gain exp. Exp is used to make your Pokemon level up. However, you should train all of your Pokemon evenly.
Gyms: These areas are kind of like actual gyms, where you train your Pokemon against trainers. At the end of the gym is a gym leader. They are strong, so do your best. Once you win, you can continue your journey.
I think I may have given too much detail, but I think these are the basics you need to get through Pokemon. However, in-game, there is a tutorial on catching Pokemon, a trainers school, and in X and Y, an adventure rule book. Hope your little brother does well!
