Geography, "geography," is the study of where things are - the location of things.  地理，“地理”，就是研究事物在哪儿——事物的位置。

It's more than that, but that's what she means here.   不止这些，但这就是她的意思。

So, Lucy's "bad at geography," and because of that, her brother "thought he would be funny" to give her "something to study while" she eats.   所以，露西的“地理不好”，正因为如此，让她“在吃饭的时候学习”，他的哥哥“认为他很有趣”。 

"He thought he would be funny," meaning he was trying to make a joke - trying to be funny.  “他认为他很有趣”这意味着他试着开个玩笑——试图搞笑。

 "Of course," Lucy says, "I haven't learned a thing," meaning I haven't learned anything or I have learned nothing.  “当然，”露西说，“我还没有学到什么，”意思是我没有学到任何东西或者我什么都没学到。

All of those mean the same - "I haven't learned a thing." 所有这些意思都是一样的——“我还没有学到什么东西”。

Lucy says that she's "still hungry after" eating her sandwich and wants "some dessert." 露西说她吃完三明治后“还很饿”，想要“一些甜点”。

Dessert, "dessert," is usually something sweet with sugar, for example, that you eat at the end of your meal.   甜点，“甜点”，通常是加了糖的甜东西，例如，饭后吃的那些。

It's what we would call your last course.  这是最后的一道菜。

After your appetizer and an entree, you can have dessert.   开胃菜和主菜之后，你可以吃甜点。

It's my favorite part of the meal. 这是我最喜欢的那部分。