Get a Treecko in Pokémon Emerald

Treecko is a grass Pokémon which you receive from Professor Birch when you start the game.

Steps

Arrive at Littleroot Town when you start the game. You'll be taken into your house and your mom will introduce you to the house.
Go upstairs and set your clock to whatever time you want.
Leave your house and go next door.
Go upstairs and examine the Poké Ball on the floor.
Leave the house and head north. You will pass a little girl near the end of Littleroot.
Keep going until Professor Birch stops you from walking.
Wait until he asks you to help him defeat the Poochyena.
Examine his bag and select Treecko from the bag.
After the battle, Professor Birch will invite you to his laboratory and give the Pokémon to you (Nickname it if you want).

If you have a Miracle Seed on hand, give it to Treecko.
If your Treecko develops Pokerus, you will gain 2x EVs.
It evolves into Grovyle when it gets to level 16, which evolves into Sceptile when it gets to level 36.
Treecko's stats will always be different, depending on his Individual Values (IVs), Effort Values (EVs), and Nature.
Water, Ground, or Rock Pokémon is a good way to gain levels.