In most cases, we use oui (yes) and non (no) in French the same way as in English.
However, if someone asks you a negative question or makes a negative statement and you want to disagree with it (i.e. answer in the positive) then use si instead of oui.
Here are some examples:
- Use Oui to answer "yes" to positive questions/statements
- Use Si to answer "yes" to negative questions/statements
Note that non is always the same whether you are agreeing or disagreeing: