After you Surf through Route 124, you reach Mossdeep City, where you can battle the leaders of the Psychic-type Gym.
In Ruby and Sapphire, you can find Steven Stone in a house here and get HM08 Dive from him. However, you can't use Dive outside of battle until you get the next Gym Badge.
You can talk to a boy in one of the houses here to find out where your Secret Base is.
In another house, a Black Belt will tell you which PokeBlock color your Pokémon prefer.
Talk to a boy in town to get a King's Rock.
You can get the Super Rod in Mossdeep City from a fisherman in a house.
There is a Net Ball in a house near the fisherman who was mentioned above.
Go into the Space Center and talk to someone there to get a Sun Stone.
Go to the Gym
When you are ready, go to the Mossdeep Gym, which is full of Psychic-type trainers.