Susan, well done on the clearance which is very hard work in itself. Plants are a personal thing, I love Peony's others say they are too short lived and not worth it, they are to me.
Really you need to wander round the garden centres where most plants will be in bloom, not much good for this year but you do see what you are getting a year on they will be settled.
Collect a few packets of seed, cheap and cheerful but read the packet for outside planting, height and spread. Fill an old coke bottle with very dry sand mix in your seed then after raking over a patch of soil make a fancy pattern by slowly releasing the sand as you move the bottle around, water in and wait. You can intermingle seed but make sure you get the smallest to the front.
Not all seed takes, some takes much longer than others so patience is a virtue in gardening.
You will have some show this year but by next year you can see the gaps that need filling, a pencil and note book are always handy in my shed.
Look around the gardens near you to see what grows well and what you would like yourself, as I said it is all a matter of taste.
good Luck Frank.