Learn Spanish (Latin America): Trabajo mucho porque quiero ganar dinero. - I work a lot because I want to earn money.

Not quite! When written as one word without an accent mark, porque means "because" and is used to give a reason, just like in this sentence. When asking a question, it's written as two words with an accent: ¿por qué? ("why?").

Why is ganar in its dictionary form (ending in -ar) instead of changing like quiero does?

When you use two verbs together to say you want to do something, only the first verb changes to match the person (quiero = I want). The second verb stays in its basic infinitive (dictionary) form, which is ganar (to earn).

Where is the word for "I" (yo) in this sentence?

In Spanish, the "I" is already built into the "-o" endings of the verbs trabajo (I work) and quiero (I want). Because those verb endings make it completely clear who is doing the action, words like yo are usually dropped unless you want to emphasize them.

Does ganar always mean "to earn"?

Ganar means "to earn" when talking about money or a salary, but it is also the common word for "to win" (like a game, a race, or a prize). You can tell which one it means from the context of the sentence.