What does "strapping" mean? Being muscular, perhaps?

What's "buckle up" mean? Is this a phrasal verb?

"Buckle up" is a phrasal verb. It means to put on or fasten one's seat belt in a vehicle or aircraft. It's commonly used to instruct people to put on their seat belts! Ex: Buckle up and enjoy the ride. Ex: Johnny, please buckle up before we go.

What's "crummy" mean?

"Crummy" is an adjective that means poor in quality, dirty, or unpleasant. It's fairly common to use. Ex: The hotel they stayed in was a little crummy, but they didn't mind. Ex: I don't want to hear your crummy jokes.

What does "mad chubs" mean? Is it a common word that I can use?

"Mad chubs" here refers to being very fat or very chubby. "Mad" is an informal way of expressing "so much", "so many" or "very". The term "chubs" here means fat. This is not a common expression to use. It would be more common to say "how did I get so chubby" or "how did I get so fat" in this case.

What does "strapping" mean? Being muscular, perhaps?

Yes, that's correct! "Strapping" means big and strong, and is often used to describe boys or men that are well-built. Ex: The Smith family has three strapping sons. Ex: The male lead of the movie was a handsome, strapping young man.

Can you show me some alternative expressions to replace "Now where to begin?"

Of course! You could use "Now where do I start?" "How should I start?" "Now, where to start?" "What should I start with? "Where should I begin?" These alternatives all indicate the same meaning as "Now where to begin?" A: Tell me how you two met. B: Okay. Where do I begin? Ex: I'm not sure where to start. Ex: I need to write a paper on mitosis, but I don't know how to begin.