I would suggest following the official Tridion Training Curriculum. It is created for this purpose of introducing new developers to Tridion and also walking them through how to build a website using the CMS. It is a minimum of 3 full days and available from SDL here.
The next best thing is to find a simple website and try to re-create this site in Tridion - starting with Page Types, Schemas, and then moving onto coding. The best resource for this would be a colleague sitting next to you to show you how to do this and walk you through these steps. This proces will take anywhere from 3 days to 2 weeks, depending how much time you put in and how complex the site is. That should cover the basic features of Tridion. However, the Tridion CMS suite is getting broader and adding new products on every year. To go into more advanced topics such as Web Service APIs and automation it can take anywhere from 3 weeks to 6 weeks to have a good understanding of all the pieces and how it fits.
What I see is that new developers have a much harder time with the concepts (Pages / Component Presentations / Broker / etc) than the technical programming parts. For this reason it is most helpful to follow the official Tridion training course or have a colleague walk you through all the steps.
Then, after this, you can follow the Tridion documentation or ask here on StackOverflow for more specific questions.