How can I prove that I didn't plagiarize?
There are a few steps you can take, but the specifics depend on the individual institution and its procedures:


Find out what evidence is behind this accusation. Presumably when you talk to this "academic integrity team," they will tell you more specifically why they think you cheated. Once you have a chance to find out what their evidence for this accusation is, you will be in a position to show them how they made a mistake. 
Find out what the institution's official policy is for appealing accusations of academic integrity. This depends on the individual institution's policies. When you speak to the "academic integrity team," you can ask them directly what the appeal procedure is.
Find out what the general grievance procedure is. If you don't have a good experience with the "academic integrity team" (e.g. they did not give you a fair chance to appeal the accusation), you should find out what procedures your institution has in place for voicing grievances in general, and you can pursue these if necessary.