At startup the engines start automatically on fmc. How do I turn that off and start the engines myself?

Load the cold and dark state using the Menu in the FMC and then start the engines using the manual or one of the tutorials/checklist you can find on this forum. You can make the cold and dark state default using the same menu.

Go into the aircraft settings under MENU -> PMDG Options. Select PANEL STATES, and in there you'll see the options for Cold/Dark, and turns. You can also tell it to load the aircraft using those states when the sim loads.Read the introduction. It tells you about all of this.