Congratulations on your pond.
You say you have a prefomed pond so the first thing you will need to do (if you haven't already) is create an entrance/exit point for wildlife. Most preformed ponds have steep sides and you will have to creat a slope by filling one side (probably from the shallow shelf to the edge) with stones, bricks... anything that can give a place for toads, newts to crawl in/out. If you know anyone with an established pond a jam jar full of the mud from the bottom of it would help get yours off to a flying start.
There are 3 types of plant that you can use in ponds. Firstly oxygenators - totally submerged in the pond and they help to oxygenate the water. eg pond weed Potamogeton
Marginals - they sit in shallow water on the shelf of your pond. eg Marsh marigolds - Caltha palustris. Be careful as these plants come in all sorts of sizes and you need a small one; a small iris -; Bowles Golden Grass.
Floaters - leaves float on the surface of the water and roots remain below. eg Water lily - you will need a pygmy one as normal ones will get very big.
Unfortunately, it is not a great time of year to buy pond plants as most of them start to die back in autumn and the garden centres will probably not have a good range.
As your pond is small you will need to go for small plants and choose carefully or you will be forever pulling out/dividing plants.
Around the edge of your pond the soil will not be wet but if you recognise where your pond over flows when full you could make a bog garden and be able to plant bog plants..
You will want ground cover that will not drop too many leaves into the water and yet give good cover to wildlife and pond edge - Euonymus fortunei 'Golden Tip'; Geranium macrorrhizum; Ajuga
I hope this helps - I also hope I haven't offended you as you may know a lot of what I have written.