How do you usually use "hold on"?

What does "bleeding sky" or "bleeding [something]" mean? Is it common metaphor? It sounds creepy.

This isn't a common metaphor, no! Here, I believe it means either the sunset or sunrise when it looks like the colors are bleeding in the sky, and sometimes the colors are quite red. "Bleed" doesn't always relate to blood, though. Sometimes liquid, gases, or other things can "bleed;" it means something seeps into something else. Ex: The colors are bleeding into each other since the paint isn't dry yet. Ex: At sunset, sometimes it looks like the sky is bleeding.

What does "nigga" mean?

"Nigga" is a racial slur. I do not recommend using this slur. Many African Americans may find this extremely offensive. It is better to avoid using this word.

Are there any words skipped in the phrase "picking up"?

Yes. "My dry cleaning" is omitted here. Since Tom is walking in to a dry cleaning shop, one can figure out that he is either dropping off or picking up his dry cleaning. This happens often in English. If something seems to be understood, many will just omit the rest of the sentence. A: How many doughnuts do you want? B: One please! (We know that Person B wants doughnuts, so the word doughnut doesn't need to be said again)

What does it mean by "heart is borrowed"?

My guess is that it means her heart may have belonged to (she gave her heart away) others in the past, hence why she says it was "borrowed" here. A similar expression used to express that one loves another is to say that one's "heart belongs to another." "My heart's been borrowed" probably means something similar, that her heart has belonged to others in the past.

"Opportunity is lost" sounds a bit unfamiliar to me. Does it mean like "you missed this opportunity?"

Yes, an opportunity that is "lost" is a chance that has been missed. Ex: I lost the opportunity to attend university because I didn't study hard enough. Ex: We didn't have time to visit the movie premiere so the opportunity to meet Brad Pitt was lost.