Documentation of processors
The datasheet from the manufacturer will tell you what's required.

The datasheet from Broadcom isn't available without signing an NDA. That's not the largest of your problems, because the device itself isn't available in small quantities either.

To support people building products around the Pi ecosystem, there is the Raspberry Pi Compute Module.

If you insist on building a board from scratch (not a beginner project), you could consider the Freescale iMX series which have more readily available documentation and run Linux.