You are looking in the right place! You have perfectly described the most common use case for Cytoscape. Fortunately, we have a ton of tutorials cover this at tutorials.cytoscape.org (https://github.com/cytoscape/cytoscape-tutorials/wiki). I'd recommend starting with the three top recommended exercises:

• Basic data visualization
• RNA-seq data analysis
• Differentially expressed genes

This page also has presentations introducing Cytoscape and dozens of resources for network files: https://github.com/cytoscape/cytoscape-tutorials/wiki#introduction

In terms of expression data, you would get those data from the regular places you find expression data: SRA, EBI atlas, TCGA, etc. Cytoscape can read any of the tabular data formats you would download from there. If you are interested in single cell RNA-seq data, there is even an app to help you browse EBI's and CZI's atlases: https://apps.cytoscape.org/apps/scnetviz