I added my username to /etc/sudoers but using sudo still asks for password I entered my username in the sudoers file, but when using sudo in scritps, it still asks for the password.

This is my sudoers file:  

~: ls -al /etc/sudoers
-r--r-----  1 root  wheel  1297 Aug 14 11:44 /etc/sudoers  


Contents:  

...
%admin  ALL=(ALL) ALL
petruza ALL=(ALL) ALL


(http://pastebin.com/raw.php?i=F38RxeYP)
(I also tried with %petruza but didn't work either)

I have a script to start XAMPP, but it asks for the password anyway:

sudo /Applications/XAMPP/xamppfiles/xampp start
Please realize that disabling the password prompt for sudo is a huge security concern. 

However, it is possible. Instead of what you already have, add this to your /etc/sudoers file:

petruza ALL=(ALL) NOPASSWD: ALL

Here is some more reading on this topic.