SPSS tutorials: chi-square (Kent State University)
This tutorial from Kent State University shows how to produce and interpret a chi-square test with two categorical variables.

Significance tests (Khan Academy)
This unit from the Khan Academy explains the concept of significance tests and error probabilities.

Inference for categorical data using chi-square tests (Khan Academy)
This unit form the Khan Academy explains the concepts behind the chi-square test for a relationship between two categorical variables.