Preface:

SugarCRM is open source Customer Relation Management application written in php(html and Javascript is also used somewhere).
It uses mysql as backend database.
http://www.sugarcrm.com/crm/
Here you can take a general idea about the Sugarcrm.

Before starting coding take a good book of php and read it thoroughly.

1) To set the working environment on local machine you need:

i) One computer with RAM-256 (512 is better)
ii) Operating System : Windows XP or Linux (Fedora 2)
iii) MySQL-4.0.x == database server, http://www.mysql.com/
iv) Apache-2.x == Web server, http://httpd.apache.org/
v) PHP-4.3.x == Scripting Language , http://php.net/
vi) Eclipse-3.x == IDE (to edit and write php files) , http://www.eclipse.org/
2) Internet connection, preferred Broadband
3) If you go with XP, then you have to install (iii, iv, v) seperately and if you prefer Linux (Fedora-2) then
all are installed automatically. But for Linux you have to learn to use it. (not difficult but not so easy either)

4) Once your system is ready with above softwares you can download Sugarcrm(http://www.sugarcrm.com/crm/)
and install in your machine.
5) SugarCRM comes with all source code so you will get all files.
6) Now start the actual coding means modification and adding extra features the customers want in crm.