Pokémon Diamond and Pearl/Pastoria Great Marsh

Similar to the Safari Zone of previous games, the Pastoria Great Marsh contains many Pokémon that are unobtainable elsewhere at this point in the game.

Visit the Observatory
Before you enter the Great Marsh, visit the observatory upstairs. The people there can tell you how the Great Marsh works, and the binoculars by the windows will give you a preview of the Pokémon that you can catch in the Great Marsh. The binoculars will usually reveal the Pokémon of the day, which does not normally appear in the Marsh, but can be caught anytime that day.

Play the Safari Game
For 500 Poké Dollars, you are allowed to take 500 steps and throw up to 30 Safari Balls. You can't fight Pokémon here, so the only way to make them easier to capture is to throw bait or splatter them with mud. This may drive the Pokémon away, but if successful, the Pokémon becomes easier to capture.

Get HM 05 (Defog)
Speak to the Safari Game player near the gate. She has found HM 05, and would be happy to give it to you. You'll need Defog later, so be sure to challenge the Safari Game at least once!

(Grez2K9 says): you dont need it straight away because pretty much the only place you need it is in celestic town and mt cornet but they arent hard to get through even without defog but a place you will need it is when you get the national dex and a new 'spring path' opens up you can find giratina in there and its very foggy and its like another dimension in each room coz theres no set path you go from 1 room n end up in a completely different 1 no the next 1 over so you will need it in there coz its hard enuff even with it so take it there or youll be in trouble  p.s. Giratina is a dragon/ghost type at level 70 it woyuld be wise to take a normal type pokemon for when he uses shadow force coz its like fly dig etc he disapears den attack with POWER on the next turn and ghost moves dnt affect normal type so take 1 with you :)