how to get string before the second slash as a parameter??

I'm working on a project ...

I want to have my institutions and get the string before second slash as a institution username

for example:

in this path:

http://married.org/institution1/class/method/

I want to get the institution1 as a parameter to detect this institution and show this it's page and informations.

so I want to write my controller between second slash and third slash

and between third slash and fourth slash I want to write my method and parameter comes after them...

what do I do??

I guess should user the htaccess, but don't know how to do!