I guess you were not given an opportunity to explain yourself because of a previous accusation. So, what is your defense. You didn’t plagiarize that much but you were not told a small amount was acceptable. You say you didn’t really enjoy the class. So what—some classes may be interesting, but the majority will be ok. College does not guarantee a good time unless it is stated on the syllabus. You say you didn’t do your best and slacked off You should expect your grades to drop or you were insufficiently vigilant about plagiarism.
I am unclear how you think you can defend yourself. Instead, find out the appropriate person to talk to, perhaps the person who takes care of judicial affairs or dismissal from school. Find out exactly what you must do and not do. Perhaps even getting a better understanding of plagiarism and appropriate referencing.
You don’t say who you are speaking to, but if it is the person who decides on punishment find out your options for the current incident and what you need to do to stop academic misbehavior. I don’t think faculty or administration want to ruin your whole life. They want you to act honestly as you pursue your future. Listen and take their punishment and suggestions. No silly excuses. Be an adult. Tell your parents what happened and the repercussions. Expect them to be justifiably angry for a large stretch of time. Like George Washington, you did it with you own little hatchet.