Match the size to your lens thread first, filter second. A step-up ring is always written as [your lens thread] then [the larger filter].
For example, if your lens has a 62mm thread and you want to use 67mm filters, choose the 62mm–67mm ring.
You'll find your lens thread size printed on the front rim or inside of your lens, marked with a Ø symbol, like Ø62mm.
(The "Ø" just means diameter.)
Still unsure? Measure the inner diameter of your lens's filter threads in millimeters. That's your first number.