A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
Celestic Town is a tiny town that preserves the history of Sinnoh and the old ways of life. The center of town has a small shrine that dates back to ancient times. This town is the hometown of Cynthia's family. Next to the Pokémon Center is Cynthia's grandmother. Talk to her and she'll tell you about the Galactic Grunt guarding the entrance to the ruins, so head down the stairs and battle the Grunt at the north end of town.
Trainers with a Vs. Seeker by their names, when alerted for a rematch using the item, may use higher-level Pokémon.
After the battle, Cynthia's grandmother will thank you, and you'll give her the Old Charm. Head inside the ruins and examine the mural at the back of the room depicting Azelf, Uxie, and Mesprit. Team Galactic's Boss Cyrus will show up, interested in the old Sinnoh legends, and after he and Cynthia's grandmother exchange their views on the current state of the world, you'll battle him.
This battle certainly shouldn't be the hardest you've yet had. Cyrus's Sneasel, although fast, has low defensive stats. A Fighting-type move is pretty much guaranteed to take it out in one hit. It's worth noting that Cyrus's entire team is weak to Rock-type moves as well.
After the battle, Cyrus will disappear in a flash and you'll be rewarded HM03 (Surf) by Cynthia's grandmother. Once you leave the ruins, you'll meet up with Cynthia, who heard what happened and now understands that Team Galactic is more dangerous than she initially thought. She'll mention there is a library in Canalave City with books detailing Sinnoh's history, west of Jubilife City. Since it is the location of your next Gym Battle, you will need to head there eventually.
Although there is no Poké Mart in the town, an elderly couple run a shop out of their house. A man standing in the shop gives away three pairs of glasses to you depending on the time of day. He gives away Choice Specs in the morning, BlackGlasses during the day, and Wise Glasses at night.
On Route 213, use Surf south of Dr. Footstep's house and follow the rock wall eastward. Eventually, you will find a narrow passage to a patch of sea that contains most of the trainers and a few items. South of that is another passage leading to Swimmer Shetlin, a Big Pearl, and a Max Revive. Whenever you visit a small island, be sure to use the Dowsing Machine on your Pokétch to find more items.
From Floaroma Town, go north to Route 205 - now that you can use Surf, you'll be able to reach Fuego Ironworks. Starting from the bridge near the entrance to this route, follow the river northwest and eventually you'll reach Fuego Ironworks. Get off at the northern bank of the river. It's worth noting that outside of the Ironworks, you can find wild Magmar.
Inside Fuego Ironworks your objective is getting to the boiler in the center, where the owner Mr. Fuego is. You will need to use the spin tiles to reach him. First head into the direction of the southeastern corner of the Ironworks at your own pace, but avoid the tile near Mr. Fuego as that one will take you through another tile that will cause you to start over. After accessing the (↓) tile in the southeastern area you will see three (←) in a line that lead to a larger patch of tiles. The northernmost (←) is the correct one to use and will get you to the area just to the right of the entrance to the boiler place.
Stars for Shards
Mr. Fuego will give you a Star Piece when you first meet him. From now on, you can trade a Star Piece for one of all four different-colored Shards.
Once your business is complete in Floaroma Meadow, Fly to Sandgem Town and use the south exit to reach Route 219. It is the first of a chain of routes that you can use to reach Sinnoh's Pal Park. The Pal Park will be inaccessible until you have the National Pokédex, but the training you'll get on the way there is valuable, and the Pal Park can be flown to after you visit it once. You'll want to bring Grass- and/or Electric-types for the trip since the Trainers on the upcoming Routes use mostly Water-types.
Though not as simple as Route 219, Route 220 is pretty straightforward. Head down the eastern side when the route forks in two, fight the Swimmer there, then backtrack and head down the western fork. A little further on, the way forward forks in two again, this time around a largish island. Grab the Max Repel just north of it, then head down the southern fork until you reach the island with the submerged shoreline. Continue east past the Carbos, making a quick detour into route 221 to fight the female Swimmer, then backtrack and head north where the Carbos is, head west briefly to fight the male Swimmer, then go east to Route 221.