Oh, that? I'm pretty sure you have to meet up with the Plasma rejects on the hill at the back of town then kick Clay's ass (the Driftveil Gym leader). That should advance the plot.
If you're having trouble with that general part of the plot, I'd recommend you look up an FAQ rather than ask here. It'll save you time and the answers will most likely link to one anyway. May as well cut out the middle man.
