Taken from the Star Wars Wiki

Dooku was a Jedi Master who fell to the dark side of the Force and became a Dark Lord of the Sith, known as Darth Tyranus. Born into a noble family on the planet Serenno, he was the heir to vast wealth and the noble title of Count. Dooku was taken by the Jedi Order as a child and apprenticed to Thame Cerulian. As a Jedi Knight, he took Qui-Gon Jinn as his first Padawan, and later trained Komari Vosa. Dooku was a respected instructor in the Jedi Temple and one of the most renowned swordsmen in the galaxy, surpassed only by Grandmaster Yoda and equal to Master Mace Windu.

Dooku spent nearly seventy years as a Jedi, but a disastrous battle on Galidraan shook his faith in the Order and the Galactic Republic. He fell under the influence of Supreme Chancellor Palpatine of Naboo, and left the Jedi Order. After learning that Palpatine was secretly the Sith Lord Darth Sidious, Dooku turned to the dark side and became a Dark Lord of the Sith himself, replacing Darth Maul as Sidious's second apprentice. His new master bestowed upon him the Sith title of Darth Tyranus.

Reclaiming his birthright as Count of Serenno and his vast fortune, Dooku conspired with Sidious to force the galaxy into a war that would bring the Sith to power. He forged ties between corporations and planets discontented with the Republic, and became the leader of the Separatist movement that coalesced into the Confederacy of Independent Systems. Thousands of star systems flocked to Dooku's banner and seceded from the Galactic Republic. Dooku also was tasked by Darth Sidious to recruit a suitable host for their clone army. He set a competition, by marking his former apprentice, Komari Vosa, now turned evil, with a 5 million credit bounty, attracting Bounty Hunters Jango Fett and Montross, with Fett ultimately winning the competition.