After defeating Misty, your next destination is Vermilion City. To get there, you'll need to go through Route 5 and Route 6. From Cerulean City, head south. You'll find a house that was previously blocked by a Team Rocket Grunt. Now that you've dealt with the Team Rocket incident in Cerulean City (after Misty), the path is clear.
Route 5 and 6 are relatively short routes with a few trainers and wild Pokémon like Meowth, Abra, and Oddish/Bellsprout (depending on version). The main challenge in Vermilion City will be the Gym Leader, Lt. Surge, who specializes in Electric-type Pokémon.