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You find Route 214 and battle some people there. After that, you will pass Valor Lakefront and you will reach Pastoria City and you can go challenge the gym leader there, Crasher Wake, he's a user of water type Pokemon. Use electricity against water. Its super effective.

Go south all the way to Pastoria City. The gym there is Water themed. After that go into the Great Marsh (Sinnoh's Safari Zone) and talk to the dude in/near the bog on the right. He'll give you HM Defog. Now back out you'll see a grunt. Chase him East, battle Rival. Continue to pursue the grunt northerly back to Veilstone. Battle him. Then go near lake Valor to meet up with Cynthia. She'll give you Medicine. Use it on the Psyduck you saw ages back north of Solaceon. Use Defog and navigate to Celestic Town. Battle the grunt (Cyrus in Platinum's case) outside the ruins on the north side. Enter ruins, look on wall. After talking to Cynthia's nan, heat back to Hearthome Gym and battle Fantina (Ghost specialist)