Thanks a lot, Justin:

1.
I don't have problems getting in [github.com] in a browser.

2.
When I try

brew install netpbm

I get the SSL certificate error message:
==> Tapping homebrew/core
Cloning into '/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core'...
fatal: unable to access '[github.com];: SSL certificate problem: Untrusted root certificate
Error: Failure while executing; `git clone [github.com] /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core --depth=1` exited with 128.
Error: Failure while executing; `/usr/local/bin/brew tap homebrew/core` exited with 1.


3.
I don't remember exactly what but I added something manually in the .bash files. I just saved .bash_profile and .bashrc with different names, followed all the steps you told me, restarted my laptop and tried to install AFNI again and got the following message:

Installing Homebrew packages

Cloning into '/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core'...
fatal: unable to access '[github.com];: SSL certificate problem: Untrusted root certificate
Error: Failure while executing; `git clone [github.com] /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core --depth=1` exited with 128.
Error: Failure while executing; `/usr/local/bin/brew tap homebrew/core` exited with 1.
==> Tapping homebrew/core
Cloning into '/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core'...
fatal: unable to access '[github.com];: SSL certificate problem: Untrusted root certificate
Error: Failure while executing; `git clone [github.com] /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core --depth=1` exited with 128.
Error: Failure while executing; `/usr/local/bin/brew tap homebrew/core` exited with 1.
Something bad happened. Try again...

4.
I also tried to install AFNI using a new user account in my laptop and unfortunately I got the same error message.

Is it possible that my system is too old for this homebrew to be install? or I wonder if I could manually add this "untrusted" keychain or certificate.
I'm clueless. Any suggestions are very welcome!