Strike While the Iron Is Hot

To act at the right moment when circumstances are favorable

Don't Judge a Book by Its Cover

Do not judge something by its appearance alone

Steal the Show

To attract the most attention and praise at an event

Long Story Short

Used to indicate that you are giving a brief summary

Storm in a Teacup

A lot of commotion or fuss about a trivial matter

Cost an Arm and a Leg

Something that is very expensive