Qǐnɡ dàjiā zhùyì yíxià.

请大家注意一下。

May I have your attention, please?

Notes:

请(qǐnɡ): please

大家(dàjiā): everyone, you(plural)

注意(zhùyì): to pay attention

一下(yíxià): for a while

Example:

A：请大家注意一下。

     Qǐnɡ dàjiā zhùyì yíxià. 

     May I have your attention, please?

B：好。

     Hǎo.

     Ok.