“下面，我给大家演奏一首钢琴曲”用英语怎么说？？

So...let me play this piano for all of u. 说话要有点幽默感。比用表示演奏一首钢琴曲，就说演奏钢琴就成。但是中间得加介词，所以 this 是比较好的。

正式的就
now ,i would like to play a song for you all.

不正式，比较口语化的可以
well,let me play a song for you .

有开场白的话，就用
so /next ,i would like to play a song for you.

Well,I am playing a song by piano!很口语但不失正式！

Now， let me play a piece for all of you．

then i will play a Pieces for all of you