你好 Lily First, let me make a correction to your question : 'but had no idea wether . . ' : but have no idea whether . . Now for the question itself : you should say 'I don't know who to vote for'. This is appropriate for both spoken and written statements. A more formal way of saying this is : 'I don't know for whom to vote'. The first way of saying this is better for most purposes. The use of 'whom' is not necessary in normal everyday speech or writing. I hope this helps you. Adrian

Thanks for your help~