Talent. Patience. Enough rejection letters to wallpaper your house with. An agent. And a stroke of luck.
On the other questions, I would recommend you seek out several books that have been written on these very topics. It's as important to know how the system works as it is important to present a good script. You will not get near the right and appropriate information here, I would suspect. And do keep in mind it's a tough as nails business to get into. Scripts and writers who want a piece of the prize are a dime a dozen.
If it something you truly want, however. Heedless of the hurdles and the failures, never give up trying.