Q:

how to create database in my salesforce's site.com and how to use it?

I'm planning to make little web on salesforce that sells e-magazines. But i don't know how to create database and use it. I'm using 30 days free trial of salesforce.
Help me.

A:

Salesforce has wrapped database engine which runs under each salesforce org. You do not need any preparation for work with it.
Salesforce database model has the following structure:

sObject is analog of database table
sObject's field is analog of database column

Salesforce has a set of default tables(sObject - Standard Salesforce objects). You has ability to create your own tables via Setup --> Create --> Object and connect these tables with standard or custom via relationships. Also you have ability to manipulate data via SOQL (SQL like query language)
I would suggest you to read these tutorials

