"i have my account in the metalink but i don't know how to start"
Start by loggin-on to MetaLink.
", if i must download the database first etc?"
Next, click the patches tab, and follow the step-by-step instructions for locating your exact release and platform.
"Parches in a Oracle 10.2.0.3.0,"
Are you taling about the CPU for this release?
Here are step-by-step directions for getting and applying patches:http://www.oracle.com/technology/support/patches.htm
"You must have a MetaLink account to download patch sets. To register for MetaLink, you will need a valid Support Identifier (CSI). Please go to http://metalink.oracle.com/
for more information.
Once an account is established, you can download patch sets from MetaLink by clicking on the Patches tab. Please contact Oracle Support by logging an iTAR at MetaLink for any questions regarding patch sets."