1. I'll explain - but I don't know where to start.
2. I'm not sure where to start this explanation.
3. There's so much I want to tell you....I'll start with the most important things first.
1. You want to say everything and are trying to think of what is best to say first.
2. You state that there is an 'explanation' and you are unsure where to start..perhaps it is a long story with many details.
3. Starting with the most important items first is a logical way to start telling someone a lot of information.
「日本のことについて話してください」と言われた場合、時間を多少稼ぐなら「① Hmmmm....where to begin?」と独り言をつぶやくくらいですね。
これは一般的に使用されている表現で、答えが幅広い時に限って使います。例えば、「日本のいいところは？」という質問を受けたら、あまりにも幅広く答えられるため、「① Hmmmm....where to begin?」と言えば、質問者側にはたくさんの情報の中から貴方が始点を探す情報処理を行っていることが伝わります。
ジュリアン
This may sound silly but I don't know where to start.
There is so much I have to say, so let me firstly start by saying....
1=This is a simple expression used to shower that you are thinking of the best way to start the conversation or to share your thoughts.
We get to use 'shall' to express an intention.
2=This expression may be used when you feel a bit embarrassed by the situation and you been honest about it.
There are people who appreciate honesty and by letting them know they will get to understand better.
3=You can use this expression to show that there is a lot you about to say and at the same time you let them know where you will begin once you have decided which point you want to start with.
There is so much to tell you , I don't know where to start
I'm not sure where to begin
If you have a lot of things that you want to say but you don't know where to start , you can say:-
1. There is so much to tell you, I don't know where to begin.
Example:- There is so much to tell you about my engagement party, I don't know where to begin.
2. I'm not sure where to begin
Example:- You asked me how and why I got fired, I'm not sure where to begin.
「話したいことはたくさんあるけど何から始めたらいいか分からない」、以下はこの状況で使うことのできる表現です。
1. There is so much to tell you, I don't know where to begin.（話したいことがありすぎてどこから始めたらいいか分からない）
例：
There is so much to tell you about my engagement party, I don't know where to begin.（婚約パーティーについては話したいことがたくさんある、どこから始めたらいいか分からない）
2. I'm not sure where to begin（どこから始めたらいいか分からない）
例：
You asked me how and why I got fired, I'm not sure where to begin.（どうして首になったのかは、いろいろあってどこから始めたらいいか分からない）
There are lots of important things for me to tell you but I can't figure out what to start with.
There is a heap of serious information that I want to give you but I don't know where to start.
I have so much to say to you but I don't know where to start.
If you have lots of important things to tell someone, you just may not be able to choose or prioritize which one. You have to quickly 'figure out' (come up with a solution) as to which topic to prioritize since they are all important. It is a question of prioritization. The expression 'a
heap of serious information' means 'a large amount of very important information'.
So, you may say:
There are lots of important things for me to tell you but I can't figure out what to start with.
or
There is a heap of serious information that I want to give you but I don't know where to start.
or
I have so much to say to you but I don't know where to start.
たくさん大事な話があると優先順位をつけるのは難しいですよね。'a heap of serious information' というのは大量の重要な情報の事を言います。
例
There are lots of important things for me to tell you but I can't figure out what to start with.
（たくさん言いたい事はあるけれど、どこから始めたらいいか、わからない）
There is a heap of serious information that I want to give you but I don't know where to start.
（伝えたい重要な情報がたくさんあるけど、どこから始めたらいいか、わからない）
I have so much to say to you but I don't know where to start.
（言いたいことはすごくいっぱいあるのに、どこから初めたら良いか分からない。）
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