You must take accurate measurements.  However, in many cases (99%) alterations will be needed.

Bust - Measure the fullest part of the Bust.  Please do not use your bra size.

Waist - Measure your waist a little above the belly button.

Hips - Measure the fullest part of the hip, including your buttocks. 

Hollow To Hem - Measure from the imprint of the lower neck to the top of your shoes.  YOU MUST HAVE the undergarments and shoes you plan to wear on your wedding day before you take ANY measurements.

In many cases plus sizes are an additional charge.