Is he talking about "Miranda's warning?" If so, could you explain to me briefly about Miranda's warning?

I'm confused about the phrase "get to go to". Is it different if I use only "go to" in this sentence?

Great question. To "get to" do something means that you are enabled or given the opportunity to do something. Here she is commenting about the fact that she will not be able to go to her prom. If we did not use "get to" it would not refer to opportunity. Ex: Tom gets to go to Disneyland this summer. Ex: I didn't get to sit down all day.

What does "duh" mean? Where did this expression come from?

When used at the beginning of a sentence that answers a question, the word "duh" expresses that the answer to the question is obvious or self-evident. Originally, "duh" was used simply to express the sound of a less intelligent person thinking, and this use was made popular by the classic Bugs Bunny cartoon. Eventually, it started to be used sarcastically to imply that the person asking the question is less intelligent and the answer is obvious. Today, the implication of stupidity has dropped from the meaning, and "duh" simply means that the answer to a question is obvious, but it can still be used to imply that the person asking is less intelligent if a very sarcastic tone is used. Ex: A: Did you brush your teeth? B: Duh, I know how important dental hygiene is... Ex: A: Do you know when the presentation starts? B: Duh... I'm not sure, I didn't look at the schedule.

What does "on the lam" mean?

"On a lam" is a slang phrase that means to be running away from something; to be running away from the law for committing a crime. I'm unsure about British English, but this expression isn't very popular in American English. We usually say "on the run" instead. Ex: The thief was on the lam for a long time. Ex: Many criminals are on the lam to avoid prosecution.

Is "sports" used always in plural form?

No, sports is used in the plural form when being spoken about generally, like many other nouns. Ex: I like eating fruits. Ex: I like visiting new places. You can also use it in the singular form, especially when talking about something specific! Ex: My favorite sport is baseball. Ex: I have never played a sport like tennis before, so I want to try it out.

What does "save" mean here?

"Save" in this scenario means to keep something for future use. The phrase "save the best for last" means to withhold the most important/valuable information until the very end of something. She waited to reveal The Enchantress until the end because she thought that The Enchantress was the government's best weapon. Ex: She saved her dessert to eat later. Ex: I saved my lunch for tomorrow. Ex: I have a few tips for you but I'll save them for when you need them.