So first, you want to be familiar with what a paradox is--it's something that seems false or like a contradiction, but they're actually true in a sense. an oxymoron is a contradictory phrase or statement; easy examples are "jumbo shrimp" or saying that someone is "found missing." shrimp are small, so describing them as jumbo is contradictory. if one is missing, they are unfound, so to say someone is "found missing" is contradictory.
with that in mind, try to categorize your quotes.
"beggarly riches" is an oxymoron, because "beggarly" describes someone who is poor, but "riches" mean wealth. this is contradictory.
"what a pity that youth must be wasted on the young" is a paradox, because it sounds illogical that youth, belonging to or characterized by youngness, could be wasted on young people, but it's true in a sense. young people tend to waste their youth, perhaps by ignoring opportunities or being reckless, etc.
"i can resist anything but temptation" is another paradox. it's contradictory because temptation in itself suggests a lack of resistance, so not resisting temptation is giving into temptation--and giving into temptation marks the absence of resistance. circular, confusing--there's your paradox.
"civil war" is an oxymoron in this context because being "civil" means to be cordial and respectful, and "war" in itself violates that mutual respect.
The theme the author wants to get across is to avoid condemnation of others for occurrences we can't understand at the time, because the same thing could happen to us in the future.
