Either "eto..." or "ano..."

You can use 'ano' or 'etou' they can mean 'um' or 'well'

It depends on your feeling. If you want the time to think, you can use "ano", "eto" or something. If you want to show your interest, you can say "naruhodo", "hee", "hoo" etc. But I think you can say the word "Aaa" in all situations. The pronunciation is the same as the English word "um", so I think it is convenient for you to use. (Be careful to put the accent of the word on the first "A".)