I need to ask you for a favor. It's urgent. 我得请你帮个忙，非常着急。
I'm calling to ask you for a favor. 我打电话是想找你帮个忙。
Can I ask you for a big favor? 我能请你帮个大忙吗？
Maybe. What do you want me to do? 也许吧，你想让我做点什么？