kaLet's see Luffy - we know he's a friend of Shanks, ate a Gomu-Gomu no mi and dreams of becoming the Pirate King also he like Gol D. Roger very much and that he's the brother of Portgas D. Ace
He started his crew in Fushia Town
Usopp - is Yassop's kid and likes Kaya, dreams of being a great warrior of the sea
He joins the crew in Syrup Town, after Luffy defeated Kuro and received the Going Merry from Kaya
Chopper - dreams to become a Pirate/Doctor, Doctor Hiluluk and Doctor Kureha were like family to him
Joins the crew in Drum, after Luffy defeat's Wapol, Doctor Kureha throws a weapons arsenal at Chopper for him to leave with the Strawhats
Nami - Bellemere took care of her and her (sister) Nojiko in Kokoyashin Village, she became part of Arlong crew after Bellemere's death, to one day buy Kokoyashin Village from Arlong, her dream is to make the map of the whole World
She joins the crew in Orange Town (for a time) and then re-joins in Kokoyashin Village after Sanji, Zoro, Usopp and Luffy defeat Arlong crew and Arlong
Sanji - was in a ship that was attacked by Zeff, since the crash and Zeff saving Sanji's life losing a leg in the process, Sanji worked with Zeff in Baratie and is dream is to find the all blue
He joins the crew in Baratie, after Luffy defeated Don Krieg, Zeff and the others make Sanji leave
Zoro - dreams about being the best swordman in the World to acomplishe the promise made to his friend Kuina, the sword he got from Kuina's father, after Kuina died, was Kuina's sword and one of the most precious swords of the World
He joins Luffy in Shell Town, after Luffy saves his life, fighting against Captain Morgan
Nico Robin - she's been wanted by the World Goverment since she was 8 years old (after she sunk 6 marine ships all by herself), she worked with Crocodile in Baroque Works and had many other jobs in dangerous organizations, also she ate the hana-hana no mi, she's very smart, she likes archaeology and that it's a family legacy also she can read "Poneglyphs", she dreams of finding "The Real History"
After Luffy saves her life by defeating Crocodile in Alabasta (Alubarna Tomb)
She joins in Alabasta (Alubarna) were she hides in the ship, later she reveales herself to the Strawhats and Luffy accepts her joining the crew
I'm maybe wrong about this but I think Nico Robin might be Gol D. Roger daugther
Cause I've seen somewhere that Gol D. Roger was an archaeologist and he definitevily could read and write "Poneglyphs" - ( In the “Poneglyph� of The City of Gold of Shandora Nico Robin found a little added text "I was here and I'll follow this words to the end. Gol D. Roger" - that proves that Gol D. Roger could read and write "Poneglyphs"
Any aways I'd like to see what you know about the past of the One Piece main characters and about my theory about Nico Robin being Gol D. Roger's daughter