You can say '请等一下' -- qǐng děng yī xià

请 (qǐng) is a polite way to say things, meaning please
等 (děng) means wait
一下 (yī xià) means one moment, or a while depends on the context.

You can also use the phrase '请稍等'. Its pronunciation is qǐng shāo děng.
稍 (shāo) means a bit or a little bit