A: I'm all at sea in Professor Black's lectures.
B: Are you? I like his lectures.
A: Can you give me some suggestions?
B: Be prepared for his lectures and then you'll feel better.
At sea的意思是“茫然，迷惑不解”。其英文解释为：puzzled, perplexed, or completely confused (about a subject or some task at hand)。
E.g. She tried to understand the instructions, but she was completely at sea.
E.g. When it comes to higher math, John is totally at sea.
E.g. I tried to do well in this class, but I've been at sea since we started.
give sb. some suggestions：给某人一些建议
E.g. I tried to give him some suggestions, but he brushed all my suggestions to one side.
E.g. Could you give me some suggestions on where I should go to find some good information?
be prepared for：为......做好准备
E.g. You must be prepared for disillusionment.
E.g. He was inadequately prepared for the impending examinations.