Right there are two main choices for lining ponds other than expensive techniques for large Koi ponds; P.V.C or Butyl. Personnally for that size pond I would recommend a Butyl liner, I would smooth other the inside with a layer of concrete to give a small round pond shape with no crack, sharp edges etc, than lay an underlay of old carpet or sand at a depth of 1-2", you may want to add shelves to the pond which can be done using bricks concreted over to give a nice smooth shelve for marginal plants. Then go to your local lfs/garden centre with the pond dimensions and they should be able to cut a liner to the correct size for you, once this done you fit it in to the pond.
Now the fun begins! Fill the pond with water and check for leaks, if none than the preperation for fish can begin. There are several filter systems you could go for, IMO and IME it is best to go and see what is available but I would recommend one of the Bioforces or Ecoclears but there are others like Cyclone, Ecopower and so on...........Now depending on which filter you purchase you may need to buy a pump to run it off, if you purchase a Bioforce for example this likely to be the case.
Now when all the filters are in and operational and the water churning around nicely you should let it settle for at least 10 days at which point I would add some plants, a pygmy Water Lily like:
N.pygmea 'Alba'
N.pygmea 'Helvola'
There are others like
N.laydekeri 'Fulgens' which is a nice red/pink and ofcourse there are more.........
Next I would consider some Marginals and Floating plant species, some of my favourites are:
Caltha palustris 'Marsh Marigold'
Or and Iris species like:
Iris pseudacorus
Iris versicolor
Floating plants:
Pistia stratiotes 'Water Lettuce'
Eichhornia crassipes 'Water Hyacinth'
Then the all important oxygenators like;
Lagarosiphon major
Then leave the pond another 10 days with everything working and going smoothly before you begin adding the fish!
For a pond of that size I would recommend:
1xGolden Tench (
Tinca tinca)
1xGreen Tench (
Tinca tinca)
8xGolden Rudd (
Scardinius erythrophthalmus)
6xGolden Minnows (
Phoxinus phoxinus)
4x Stone Loach (
Nemacheilus barbatus)/Weather Loach (
Misgurnus angullicaudatus)/Gudgeon (
Gobio gobio).
Stone Loach (
Nemacheilus barbatus)
Weather Loach (
Misgurnus angullicaudatus)
Gudgeon (
Gobio gobio)
You could also add 2-4x
Carassius auratus, obviously you would chose the variant, be it Comet or Shubunkin.