It's the carbs you are counting, so if th bread pack says 15g per slice then 2 slices would be your 30g.
Most packs of food show the carbs per 100g or per portion, slice etc. Didn't your dietician give you some leaflets/booklets to help you? Bad show if she didn't!
You'll soon get used to counting carbs. There are some things you'll have to look out for, especially when you start insulin. For example, make sure you know whether the packet is telling you the carb values of raw or cooked rice or pasta.
Any problems, just ask and someone will try to solve them for you.
Oh, and congratulations on your great news!