This is my team for Platinum. I need help deciding items. I don't care about natures.
Torterra @ ?????
Wood Hammer
Earthquake
Rock Slide
Leaf Storm
Octillery @ ?????
Psychic
Surf
Waterfall
Signal Beam
Houndoom @ ?????
Crunch
Overheat
Fire Blast
Sludge Bomb
Altaria @ ?????
Dragon Pulse
Fly
Aerial Ace
Dragon Breath
Magnezone @ ?????
Thunder
Thunderbolt
Flash Cannon
Magnet Bomb
Froslass @ ?????
Ice Beam
Shadow Ball
Wake-Up Slap
Avalanche
First of all, natures. I know you don't care about them, but it would be beneficial if you didn't get a Pokemon with a detrimental nature. In-game, you really don't need to care about specific natures as much, but you at least don't want, say, an Adamant (+Atk -Sp.Atk) Alakazam (or any other Sp.Attacker).
Secondly, the team. Altaria and Octillery, are, to be frank, not good. I would not recommend Altaria at all, as it is frail and it will go down before you can have it do anything good. If you need a Flying type, I would be more inclined to suggest a Staraptor. Trust me, even though it's a basic "Starter Bird", it packs an insanely strong punch (literally, too, it can learn Close Combat), and it is easy to capture early on. Really, capturability is the main problem of your team. Besides Torterra, you must get to Veilstone (at least) to start obtaining your Pokemon, so your Turtwig/Grotle/Torterra will be horrifically overleveled when you start getting your others. Anyways, Octillery. Honestly, this isn't nearly as bad as Altaria, and you could easily make this guy work. There aren't many good Water-types in Platinum, so I'd have to say this isn't too bad, but I would much rather have, say, a Vaporeon than this octopus.
So then, items. Torterra seems to be Grass-oriented, so a Miracle Seed would work well. Someone should have Leftovers, they are always good in-game. Honestly, though, I don't have many suggestions on items, because they simply aren't as important in-game as they are in competitive play. For in-game, for the most part, type-boosting attacks are fine, but if you want to go all-out and get some Life Orbs and Choice Specs in there, fine.