"I need to do something."
Translation:Muszę coś robić.
Do you mean "Muszę coś zrobić" ? It's hard to tell with such ambiguous sentence. It feels like a dictionary. Other than robić is focused on activity and zrobić on result. All hipothetical situations I can imagine "I need to do something " , "Muszę coś zrobić" seems more probable.
All hipothetical situations I can imagine "muszę coś robić" it seems strange and "I need to be doing something= I need to be busy" seems like a translation.
But somebody does something annoying "do you have to do this?" - "Musisz to robić?"
Dictionaries are very helpful, but not always. They may show the meaning of a single word, but not always its use in real language.
In general, when English speaker says "I need to"/ "I have to"/
"I must", the Polish speaker says "Muszę".
I need to do something - Muszę coś zrobić
I need to keep myself busy/occupied - Muszę coś robić
to need to - musieć
to need - potrzebować
The use of the verb to need differs from the use of the Polish verb potrzebować. It is a an ERROR to follow it by another VERB in Polish.