Hi Tolga,
Welcome to the forum. Firstly I would strongly recommend that you get a good MS CRM book to help you on your way to learning MS CRM.
The answer to your questions below:
1- To customize the navigation of the application you will need to modify the sitemap. You cannot add a new tab but you can customize the existing tabs and add links to it.
2- You will need to customize the account entity to add custom attributes to the account record type.
Settings - > Customizations -> Customize Entity ->Select and double click the record type you want to modify -> To add tab navigate to the attributes area or to modify form or view navigate to forms and views. After you've made the change dont for get to publish your changes.
Hassan.
Hassan Hussain | http://hassanhussain.wordpress.com/