In an oral conversation, the most idiomatic way to express that you didn't understand something is to say « Pardon ? » [paʁ.dɔ̃], with a rising tone of voice (the rising tone is the way to convey a question in French). Merely saying that in a conversation conveys “I didn't understand what you just said”, and will usually cause the person who just spoke to repeat.
If you want to explicitly ask for repetition, you can ask “Pardon ? Pouvez-vous répéter ?” [pu.ve.vu.ʁe.pe.te] (with a rising tone of voice for both sentences). If it's someone you say tu to, the sentence is “Peux-tu répéter ?” [pø.ty.ʁe.pe.te].
Additional sentences may be useful in this context.
Pardon ? Pouvez-vous répéter plus lentement ? (Please repeat more slowly.)
Pardon ? Je n'ai pas compris le mot après […]. (I'm sorry, I didn't understand the word after […])
Je suis désolé, je ne comprends pas. (I'm sorry, I don't understand.)