Well, the first thing you should do is to get your starter pokemon. Which are Turtwig, Chimchar and Piplup.
I'll explain to you each starter.
CHIMCHAR:
Type: Fire
Pros: Very fast and evolves into a Fire/Fighting type.
Cons: Has a type weakness on the first and fourth gym.
TURTWIG:
Type: Grass
Pros: High ATK stat and DEF stat.
Cons: Very slow.
PIPLUP:
Type: Water
Pros: Evolves into the defensive Empoleon.
Cons: Very slow.
After that go and earn your first badge so you can enable the trading option on the pokemon center. The first gym leader is Roark.
After that, be sure to learn weaknesses and resistances.
Some tips:
1. Be sure to catch a Starly and a Shinx when you encouter it after you got your pokeballs. Starly's pretty weak at first, but after it evolves into Staraptor, you won't regret it. Same goes to Shinx. When it evolves into Luxray, that Eletric pokemon will own all the other eletric pokemon.
2. Get some Potions. It's very useful in case if your pokemon is low on HP.
3. Get some status healers (ex: Antidote, Paralyze Heal, etc.)
4. Never run away from a battle, except if your pokemon is in a verge of death or the opponent's pokemon is way too powerful for you (10 levels above you.)
That's all I can say for now though...