A few simple Google searches would point you in the right direction. Some useful resources are as follows:
Magento for Developers: Part 1 - Introduction to Magento
This covers an introduction to Magento, the Magento config, controller dispatch, layouts, blocks & templates, models and ORM basics, setup resources, advanced ORM (EAV) as well as varien data. (In part 1 - 8).
Another great place to feed your brain is:
Alan Storm's Blog: Magento Articles for Professional Developers
Magento's site also offers a whole bunch of downloadable resources:
User Guides and eCommerce Tips
Including user guides, designer's guides as well as other useful resources for maintaining a successful e-commerce site.
And last, but certainly not least (and this one in particular I knew about before starting any form of development with Magento - note: this is a direct .PDF download):
Conquer the 5 Most. Common Magento Coding. Issues to Optimize Your Site for Performance
What you need to understand is that Magento is not something you can "learn" everything about in a week, month, or even a year. It is an amazing platform with literally endless extendability and will involve life-long learning :)
There are plenty more resources available online, you'll need to have a look further for yourself. The Magento StackExchange in particular is your bread and butter when it comes to diagnosing problematic code, be sure to read these pages:
How to Ask
I can assure you, over 50% of the questions you may have are likely already answered here. So be sure to make use of the site's wonderful search function.
Good luck & have fun.