First You have to learn the Qweb template Engine from the below link and learn the QWeb Report from the  lunch module in ODOO 

https://doc.odoo.com/trunk/web/qweb/

http://odoo-80.readthedocs.org/en/latest/reference/reports.html

Then After You should start  learning with sale ,crm and account module report for more

Hi,

Following link of code may help you :

https://code.launchpad.net/~openerp-dev/openerp-web/trunk-qweb-report

You can find here https://www.odoo.com/forum/Help-1/question/Is-there-any-way-to-preview-WebKit-report-layout-while-coding-maybe-WYSIWYG-Editor-52307#answer-52417 a step-by-step guide to use/edit the html reports.

There is complete guide related to reports in qweb. Hope this will help you and others (Odoo beginners).

Thanks