Get the band size by fastening the bra with the cups behind You should be able to insert two fingers under the band with minimal difficulty but not significantly lift the band. Hooks should be fastened on their loosest setting, to allow the bra to be tightened as it ages.
Once you have an appropriately comfortable band in this style, choose the cup size which feels most comfortable in that band size. If there is none, it may be that the spacing of the cups or their shape is unsuitable.
Repeat the process in a different style, if required until you find something which fits comfortably and looks correctly fitted (a smooth silhouette, without gaping or pinching into four).
A band which is too large, and cups too small is a common fault according to polled data and measuring services. Not helped by a paucity of styles in combinations of small band and large cup scales.
A C cup on a 34 should be the same volume approximately as a B cup on a 36, if they are from the same style and manufacturer. They can vary substantially from different manufacturers and between styles though.