finish the installation of the server, create database, login and go to settings and all the modules manually. thats the easiest way of doing it :)

You can choose whatever you want to install but here are some a lot of people need:

• Purchase management
• Sales management
• Warehouse management
• Point of Sale
• Accounting and finance
• MRP
• eCommerce

Those are the most used modules. Another handy tip is to unlock the technical features on your account. You will be able to do a lot more.
Go to settings > Users then open up your user and then check the mark.

The installation and adding modules depends on the OS you are using. Openerp-server by itself is nothing without its  modules that bring all the functionalities based on your needs. Basic modules are CRM, HR, Accounting, MRP etc

If you are trying in windows, once you get it installed successfully, start by creating a DB: "Create new Database". In that if you ticked Load demonstration data, once your DB is created and installed any basic module, demo data will be available to get you a better understanding of that module functionality.  To install new modules, you can goto Settings >> Apps. If that module is not available locally in your installation, when you try to install it will redirect to your odoo account and you can finish installation. You will get the corresponding menu items, views and functionalities of that module you just installed.

In Ubuntu, you can get the complete source package including all the major modules available locally. From developer point of view, ubuntu installation is more preferrable.

You can download official modules from odoo.com for version 7.0, add manually also. So based on your needs search for your modules in Apps under Settings or download and add manually.