Acrobat Reader DC is free, and can't redact files.
What exactly did you purchase?
The instructions posted by Bernd are a good introduction to the process. I'd like to add two things:
Redaction is only available in Adobe Acrobat Pro - not in Standard, and as was already explained, not in the free Reader.
Redaction is always a two step process: You first mark, and in a second step, you apply these marks to the document, and only this second step will actually remove the information you want to redact. If you forget about the second step, your document will not be redacted, and everybody will be able to see what information you've marked for redaction.