You should really learn to manage your money better.
你真的需要好好地理財了。
Somebody manages his/her money well.
某人很會理財。
Somebody can't handle/manage money.
Somebody is bad with money.
某人不會花錢，不懂理財。

Somebody is a penny pincher.
某人一毛不拔。
Somebody is stingy / cheap / tight / close-fisted.
某人很小氣。
fist 本意為拳頭，close-fisted 形容把錢抓在手里，握緊拳頭不撒手。

You should always save for a rainy day.
要存點錢，以備不時之需。
save for a rainy day
存錢以備不時之需，未雨綢繆
Older generations in America tend to be more frugal than younger ones.
老一輩的美國人會比年輕人要節省一些。
frugal 節省的，節儉的

Somebody is in debt. 某人欠錢，負債了。
若要表示程度之深，可以講成 be deep in debt. 比如：
Her husband Peter is dying of cancer and the couple are deep in debt.
她的丈夫 Peter 患了癌癥，夫妻兩人欠了大筆債。

control (all the) finances: 掌管經濟大權
A husband controlling finances can lead to relationship trouble.
一個家庭如果丈夫管錢的話，夫妻關系可能就會出現麻煩和問題。

squander one's money 亂花錢，浪費錢
In less than three years he had squandered the entire family fortune.
在不到三年的時間內，他揮霍完了所有的家產。

splurge on something 花大價錢買某物（通常說話者認為不值得花那么多錢）
splurge on a new dress
花大價錢買了一條新裙子
You should let yourself splurge once in a while.
你應該允許自己時不時地揮霍一次。