This is an edited version of what I wrote in another thread about it, should answer all of your questions (this is all from Labyrutnh of Evil):
Sifo Dyas was a confidante/friend of Dooku, also a Jedi. Both of them sensed a "coming storm" or disturbance in the galaxy. Dooku was also disillusioned with the republic and its place in the galaxy. He felt it was too ineffectual. This led to him leaving the Jedi order. Sifo Dyas did not have the nerve to follow Dooku in exploring the dark side to learn more, or be moved to any direct action to deal with this threat.
Basically both of them sensed something big was going to come down the pike. He did in fact order the clone army, sensing the republic would need it eventually. Sidious discovers this with Dooku already under his wing. He allows the clone army to stay hidden on Kamino seeing how it can meet his purposes later on but decides Sifo Dyas must die. Dooku kills his former friend and is given the title Darth Tyranus. He then erases Kamino from the Jedi records.
It's actually not too complicated.
Here is an exact passage from the book:
"One of your former confidants in the Jedi Temple has perceived the coming change, Sidious had told him. This one has contacted a group of cloners, regarding the creation of an army for the Republic. The order for the army can stand, for we will be able to make use of that army someday. But Master Sifo-Dyas cannot stand, for the Jedi cannot learn about the army until we are prepared to have them learn of it.
And so with the murder of Sifo-Dyas, Dooku had embraced the dark side fully, and Sidious conferred on him the title of Darth Tyrannus. His final act before leaving the Jedi Order was to erase all mention of Kamino from the Jedi archives. Then, he had found Fetton Bogg 4; had instructed him to deliver himself to Kamino; and had arranged for payment to be made to the cloners through circuitous routes..."
