I am trying to scan all the nearby routers IP Addresses. How can this be achieved? I am new at using Nmap and tried scanning with that. I gave initial target as my Public IP Address range but it didn't help much.
Are you talking about other people's public IP addresses or routers on your home network?
If you're attempting the former you best stop there, if you don't have permission you shouldn't be scanning ANYTHING. You can even get in trouble scanning your own public IP address with some ISPs.
If you're attempting the latter, give NMAP your private address range to scan, not your public address. Your routers most likely sit behind an ISP owned device that hands out private addresses to your local network.
Again, if you're scanning public IP addresses, have permission to do so, or you could get yourself into hot water.
I see now in your comment you have said the ones that are "within your Wi-Fi range". What you are attempting to do could be considered illegal, even if it is a grey-ish area. Just don't. Want to point your NMAP at something to get some practice? Find a bug bounty website (like bugcrowd or something similar), join, and point your scanners at the companies who are actually asking to be scanned. Or, setup a home lab environment. Scanning public addresses isn't a good idea.
Nmap actually hosts
scanme.nmap.org for testing purposes just like this one (no DoS, DDoS, or vulnerability exploitation allowed).
If you'd like to do a little reading on the subject, nmap has a page on their website dedicated to the discussion of legality and port scanning. Essentially, port scanning isn't explicitly criminal (at least in the USA), but it is an extremely complicated issue, and you don't want to be one of the outlier cases, so it's best to only scan where you've been given permission.