Don't use it. What would your reaction be if a foreigner used such an expression in Chinese? You'd feel offended, am I right?

I agree - for swearing and cursing, it's best to know about it but never use it. Plus if you have a foreign accent, swearing loses all impact and sounds silly. (I've made that mistake!!) "Holy shit!" is also used as an exclamation of surprise or shock. It's a stand-alone phrase, so no need to put it in any sentences.

It is a kind if cursing/swearing. Its is simillar in meaning to "What a mess". Literaly it is "holy" = blessed, belonging to god and "shit" = very vulgar for the human excrements Well, and do not use it if you do not know the person very well, you might hurt some religious feelings. A general (very soft) replacement could be "How ugly"