I am a 20-year-old girl. I went with a young man to his house, and we only exchanged kisses. I felt a great deal of guilt and left him. He proposed to me, but I refused. I feel deep regret and cannot sleep from overthinking. Two days prior, I dreamt that I had committed an act of disbelief (kufr), and my friend dreamt that I was walking in the market with my head uncovered and wearing transparent clothes. I am asking about expiation (kaffarah).

All praise is due to Allah, and thereafter:

Indeed, a believer magnifies a sin due to the greatness of the One who was disobeyed. However, Allah's mercy is vast, and sincere repentance is sufficient to expiate the sin. You must block every means that could lead you to fall into it again and resume your life as if nothing had happened. As for your dream of disbelief, do not pay any attention to it. Similarly, your friend's dream might be good tidings of your repentance being accepted. Regarding marriage to that young man, there is no religious impediment to it. However, if you fear for his religious commitment and character, then it is better not to agree.