One Piece is the story of Monkey D. Luffy who embarks on a quest to the Grand Line, the most dangerous of all the seas, to find the greatest treasure left by Gold Rogers, the Pirate King who was executed 20 years ago, the One Piece. What the One Piece is, nobody knows, but it is known to hold a great value.
Along the way he gathers the oddest members a pirate crew could ever by made of as they venture through the dangerous waters of East Blue before entering the Grand Line.
The anime is mainly adventure, but has some kickass fights in it as a bonus with a twisted plot revealed rather late in the anime (and the plot thickens GREATLY around the 400th chapter of the manga), so if you're looking for a good anime to watch, this is the one for you.
Now don't let the 'kiddy-ish' art turn you off, as eventually you'll get to love it as it VERY unique.
You can get the latest chapters from Mangastream
You can get the anime from Anime Media - Watch Episodes Online | Watch OnePiece | OnePiece
You can also get spoiler info from Arlong Park
I would like to note that there are fillers which you don't need to download/watch as they don't affect the anime's story in ANY WAY. For the time being I would recommend not downloading episodes 54-61
& episodes 131-138
, you CAN download them if you want BTW, since they're not as shitty as Naruto's AND Bleach's.