scan a block of addresses
 

I want to scan an isp to see how many hosts are up on that isp's network.
Im using nmap

1st. how and what ip address do i use in the scan?
2nd how can i use nmap through a proxy?
3rd is it possible to find my friends computer by only knowing his isp?

how and what ip address do i use in the scan?
Check "man nmap", then ask OK? Look for "octet range address".


how can i use nmap through a proxy?
I would advice you to not use decoys or any stealth scan options.


is it possible to find my friends computer by only knowing his isp?
Not unless you have more details about what "your friend" runs. "Your friend" might not like you scanning, or simply block scans.