You should create a custom addons directory, specify the path in the openerp-server.conf file and then upload your custom modules to this addon directory. Use the same permissions as in the original Odoo addons folder.

After that login to Odoo as admin, activate "Technical Features" or the "Developer mode" (Version 9.0), go to the Apps menu, update the module list and then you should see your custom modules in the app list. Install the apps you wish to install.

Beginner's instruction for Version 9.0:

 
- carefully study the available official documentation and additional sources (https://www.odoo.com/documentation/9.0/ and https://www.odoo.com/documentation/user/9.0/)
- Ubuntu: please use a longterm stable release such as 16.04.1 LTS
- as a beginner I recommend to use the package installer of Odoo, "Deb" version, see http://www.odoo.com/documentation/9.0/setup/install.html#packaged-installers
- when using the package installer, you'll find the openerp-server.conf file at /etc/odoo/openerp-server.conf
- adding addons paths see https://www.odoo.com/forum/help-1/question/custom-modules-folder-path-29358, just add a comma and your custom path (without leading space)
- Activating the developer mode in 9.0 (Enterprise and Community): https://www.odoo.com/forum/how-to/developers-13/how-can-i-activate-the-developer-mode-83
- Apps menu with developer mode activated: http://i1.wp.com/pulse7.net/wp-content/uploads/2015/11/odoo-9.png
General advise: Google is your best friend :-)
Please use the mailing lists (www.odoo.com/groups) and the Help-Forum for further help.
Hope this helps you to get started.