It would depend. You could say this when you are fishing, literally after you caught a fish. And you could say this when you feel you met someone who was a popular person to want to date, ''a catch''. Which even in a romantic sense, the meaning of "I got myself a catch" is still a idiomatic expression that stems from the fishing world. If someone tells you, ''you're a catch'' it means that they find you to be attractive and someone who is very successful in life.

Really depends on the context. The catch could be a person or opportunity or idea... It basically means they've done really well by getting the 'catch'. They view it highly

it simply means you found something good. English speakers typically refer to a really good partner as a "catch." for example, "i'm dating this new girl and she's a real catch."