If you are new to programming or if you have the patience to work through a textbook:
How to Design Programs is the best place to start.
Continue: Web Applications in PLT Scheme introduces you to the Module language and building web applications.
Guide: PLT Scheme describes the rest of the PLT Scheme language, which is much bigger than the learning-oriented languages of the textbook. Since you learned functional programming from the textbook, you’ll be able to skim chapters 1 and 2 of the Guide.
If you’re already a programmer and you’re in more of a hurry:
Quick: An Introduction to PLT Scheme with Pictures gives you a taste of PLT Scheme.
More: Systems Programming with PLT Scheme dives much deeper and much faster. If it’s too much, just skip to the Guide.
Guide: PLT Scheme starts with a tutorial on Scheme based, and then it describes the rest of the PLT Scheme language.
Of course, you should feel free to mix and match the above two tracks, since there is information in each that is not in the other.