Head to the left to find a hiker and a slippery slope. You need to get a Mach Bike before you can go up this slope. You also need that bike to reach the other items hidden in the game. Make sure to return here to get the items later on. Talk to the hiker to receive TM70 Flash.
Head straight down and you'll find a chamber where Steven in located. After delivering the letter to him, you'll get TM51 Steel Wing. Steven will leave shortly. Now leave the cave and return to Dewford Town.
Go to the Docks and talk to Mr. Briney, where you'll learn about your new objective. He'll also offer a ride to Slateport.
U have to beat the gym first
There's 3 guys blocking the way the way. How do I get them to move?