I found some of this on a site:
Grievous was originally an organic Kaleesh, a warlord of considerable standing with many wives and children. When his species became locked in an interplanetary war with the Huk, the Huk initially were victorious; however, when Grievous took control of the Kaleesh's forces, he turned the tide of the war. His vicious counterattacks and mass destruction of entire Huk planets ensured Kaleesh victory—until the Huks unexpectedly turned to the Republic for help.
Transformation to Cyborg
Jedi Knights were sent to resolve the conflict sided with the Huks, and hefty fines and embargoes came down upon the Kaleesh. Kalee ended up in poverty, and hundreds of thousands of Kaleesh starved and died.
The arrival of San Hill and the InterGalactic Banking Clan brought an opportunity to Grievous. In exchange for leading the Clan's droid enforcer armies, the Banking Clan offered to help the Kaleesh overcome the effects of the embargo.
At first, Grievous refused, wishing to stay and lead his people through their trying times. Eventually he agreed and for a short time became an extremely effective enforcer. Soon however his desire to aid his people resurfaced. It was then that a Separatist bomb crashed a shuttle Grievous was on, causing near fatal injuries and rendering a body that had survived many battles useless, or so he was told. Planted evidence pointed to a Republic plot. In truth Grievous once again proved his resilience and additional injuries had to be added after the crash to make his body as broken as was claimed. San Hill repeated his proposal, including a promise to replace Grievous' broken body. Desiring vengeance against the Galactic Republic and the Huk, Grievous agreed on the condition that his mind would not be tampered with.
The Kaleesh commander was taken to the planet Geonosis, where Poggle the Lesser and his Geonosian scientists implanted Grievous' brain, eyes, and vital organs into a duranium alloy body reminiscent of a Krath war droid. In addition, the Geonosians also altered his mentality and memories to make him more violent and vengeful. His blood was completely replaced by Jedi Master Sifo-Dyas' on order of Count Dooku. The cyborg project was funded by San Hill to create a military leader that could counter Count Dooku's growing political influence, although he presented the finished General Grievous as a twisted gift to Dooku. Though taken aback by his appearance, Dooku was pleased and bestowed upon Grievous the title Supreme Commander of the Droid Armies, and planned to use him in his efforts to conquer the Republic. Grievous still displayed some of his Kaleesh heritage by adorning himself and his MagnaGuards with the styles of masks and cloaks closely resembling those found amongst his people.
Dooku trained Grievous in the art of lightsaber combat, which Grievous took to at a rapid pace. His new skills were tested at the Battle of Geonosis, but none of his Jedi or clone opponents lived to tell of his existence. Despite his obvious prowess, and the time and resources expended to creating him, Dooku still viewed him as second to the Chiss general Sev'rance Tann, and it was only after Tann's death one month after Geonosis that Grievous truly began to come into his own.
Here is the site where I found it from: