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What do you need to know
Yii learning curve is not as steep as other PHP frameworks but still it requires some beforehand knowledge.
PHP
Yii is a PHP framework so make sure you read and understand language reference.
Object oriented programming
Basic understanding of object oriented programming is required. If you're not familiar with it, check one of the many tutorials available such as the one from tuts+.
Note that the more complicated your application is the more advanced OOP concepts your should learn in order to successfully manage that complexity.
Command line and composer
Yii extensively uses de-facto standard PHP package manager, Composer so make sure you read and understand its guide. If you are not familiar with using command line it is time to start trying. Once you'll learn the basics you'll never want to work without it.