My classmate copied my answer word for word, how do I make sure I don't get blamed for this?

Document what happened first, and then tell your professor about what happened and what you observed. If necessary, show him or her the documentation of your original work.

Teachers know who has the brains.  The other one has glassy eyes, they are not that smart.

I would find your professor and mention the situation to him.

Always Print your answer on a computer with the date and Time submitted

I had a student that copied other people’s answers but ran them through software to replace every word with a synonym. I thought at first they were a non-native English speaker but I caught on when Adam  Smith’s invisible hand of God literally became the indiscernible fingers of Vishnu.  I am NOT joking or being clever.  I never laughed so hard in my life giving a student zeroes retroactively on every discussion post. 

how did this person get your answer?  did you copy it from some internet source?  give it to him?

The college will know which response was submitted first

but I would still 'mention it' to someone !

As soon as you found out you should have contacted your teacher and told them what happened.

My grandson had the same thing happen to him several times (by the same student) last semester.  There were no repercussions to my grandson.  I don't know if the professor contacted the other student. The online program used by his college records the date and time of all submissions,  Consequently, it was clear my grandson turned in the assignment well before the other student.  Hopefully, you have similar proof of when work is submitted.

Isn't there a time stamp? Or aren't the answers posted in sequence? It should be fairly easy to tell which was posted first.