What's "gear up" mean?

Are "tramp" and "trudge" used commonly in a daily conversation?

No, these aren't used commonly in conversation! Although, in this context, I feel like "trudge" is probably used more so than "tramp." One can "trudge" through something or to express an unwillingness to do something in the way you walk. Ex: She trudged through the snow to get to school. Ex: You can't trudge through the store acting that way. We'll go home soon, don't worry.

What's he trying say by "worst nightmare?" It's not about dreams, right?

Yes, it's not meant literally! "Worst nightmare" is an idiomatic expression used to describe a terrible, undesirable situation - one so bad that it's like a nightmare. Ex: I'm allergic to cats so when I heard my new roommate had 10 cats, it was like my worst nightmare ever came true. Ex: It was my worst nightmare. I walked into class to find out we had a surprise exam.

What does "wanna" mean?

"Wanna" is the contraction of "want to" or "want a." In this case, it means "want to." Ex: I wanna (want to) go home. Ex: I wanna (want a) new phone.

What does "shit" mean?

'shit' is foul language. It is a rude word used "to express anger, annoyance, fear, or disappointment" and also meaning "body waste", "something (bad)", or "stupid/untrue talk". In the sentence above, it is used as "something".

What does this sentence mean?

This sentence is a part of the Pledge of Allegiance of the United States. The Pledge of Allegiance was written for U.S. citizens to recite as a pledge of their allegiance and loyalty to the United States. He is reciting this in the video here because he is trying to cover up why he put his hand over his jacket, so he pretends like he is reciting the Pledge of Allegiance because when people recite this, they stand with their hands over their hearts.