If your Ford Focus is not starting, and you know the battery and alternator are good, the problem could lie in the ignition system. The ignition switch is a whole cylinder inserted into the steering column that connects to the starter motor. When you change an ignition switch, you are also replacing the key.
Remove the dashboard panel below the steering column in your car; it should be screwed in place. Remove the steering column's upper cover by releasing its clips with a flathead screwdriver.
Look for the audio control switch---a small device sticking out of the steering column that acts like a remote control. If one exists, release the switch's plastic locking tang with the screwdriver, and unplug the electrical connector to remove the switch.
Loosen the steering column by releasing the locking lever, and remove the steering column's lower cover by pulling out the screws with the screwdriver.
Use your ignition key to turn the switch to the left---the position that would activate the accessories but not start the car. Insert the screwdriver into the hole in the top of the cylinder to depress the detent spring, and pull out the cylinder with the key inside.
Insert the new key into the new ignition switch, and line up the switch inside the steering column so it's in the same place the old switch was in when you removed it. Make sure the detent locks into place; then turn the switch so it's in the "Off" position.
Replace all the covers and panels you removed to access the cylinder, along with the audio control switch, if applicable, in reverse order of removal.
Older Ford Focus models may have a different method of removing and changing the ignition switch. Check an automotive guide for your particular year and model.
Always disconnect the negative (black) battery cable before working on the ignition or any other electrical component of your car.