What do I write as response for an academic misconduct investigation?
If I understand this correctly, you were under the impression that proper quotation of sources means to have them in your bibliography (and nothing else). You then quoted from those sources without quotation marks and footnotes to the reference quoted.
I’d explain that to them, apologize, and ask for a good source how, in the future, to quote according to their specific guidelines. That’s the truth; it doesn’t strike me as terrible in your first year, but you’ll have to hope your professors agree.