"저는 친구와 걸어요."
Translation:I walk with a friend.
걷다 is a ㄷ irregular verb. It doesn't follow the basic rule for conjugation. For ㄷ irregular verbs, the ㄷ becomes a ㄹ, and if the last vowel is ㅏ/ㅗ, add 아 to that. Otherwise, add 어.
You can look up how to conjugate basic and irregular verbs if you would like. There are irregular verbs for (ㅂ/ㅅ/ㄹ/ㄷ/르/ㅎ)
In the American English I've always used and heard, this doesn't mean quite the same thing. Here are some examples that make it easier for me to explain it:
"I walk my dog." Here the speaker is taking his or her dog out for a walk to get exercise. I don't know if it's the right way to describe it, but this one almost evokes a sense of duty on the part of the speaker. It's his/her duty to take the dog out to care for it, or he/she is being paid to take the dog for a walk.
"I walk with my friend." Here the speaker is taking a walk with his/her friend. This could be walking to a destination, like a classroom, just a stroll through the park, or they could be doing their daily walk for exercise together.
"I walk my friend to his/her car/house/classroom/etc." Here the speaker's friend is headed somewhere, and the speaker wants to accompany him/her. This could be someone who is leaving after a party, and the speaker wants to walk him/her to the door to see him/her off, it could be a boyfriend walking his girlfriend to her next class, or it could even be a police officer walking someone to his/her car for safety reasons.
Without context, "I walk a friend" sounds, to my ear, like number 1 when I think the exercise is trying to say number 2.