Most Hebrew speakers pronounce these letters the same way - /x/, and you'll just have to learn how to distinguish between them, based on context and experience. Some Hebrew speakers, mainly Sephardi and Mizrahi, do pronounce these two letters differently - כ is /x/, and ח is /ħ/. You can read more here - https://en.wikipedia.org/wiki/Modern_Hebrew_phonology
I wish I could do an audio. ח is a deep H, a more stressed one but כ (without dagesh) is a gutteral chrrr sound. The same is with א וע, alef is simply 'a' sound but ayin is a deep 'a' which needs to come from your throat... Kinda like nausea. צ is not tz sound but a deep 's' like a sssode. These are the few confusing sounds.
So, "לכן" is to a group of girls (or at least to more than one girl), and "לך" is to one person...for a guy it would be "l'cha" and for a girl it would be "lach". Also, just to throw it in with the others, "לכם" would be to a group of people. "לכן" makes it specifically to girls. Hope that helps!