+1: Get a proper training. With "proper" meaning "a training which makes you undestand things, not one which rushes you through button pressing".
Do the tutorials Gianni mention. Get yourself a SampleApp instance or at least connect online to the public ones available.
Also: Don't focus on "I know SQL so I will look at try to reflect query A which I have written in a report". OBI is a totally different concept. Everything revolves around metadata models. Not SQL queries.