Filter for pond, clueless as to what we need!

[EustachianTube]

We moved into a house with a large pond in the garden. The water is rather green so we think it'd benefit from a filter/pump but we've not a clue where to start with figuring out what we'd need. There's a lot of fish in the pond (I'd guess around 100-150?), water lilies and plenty of plant life around the edges.
I don't know how deep it is or what the approx volume is in litres so am a bit stuck with where to start. Any advice would be appreciated!

We have a pond and a filter box. The box has three chambers in and the water runs through each one. Dh has big brushes in it in the first chamber. Then he has different thickness sponges in the others. We also have a UV light which helps a lot. Sorry I can't tell you any more details as I don't actually maintain it ! The brushes are like gigantic baby bottle brushes. A pump is essential and that is contained in the third chamber. Depending how many gallons of water you have depends what pump size you will need. There is a way to calculate how much water you have, but until my Dh comes home this evening I have no idea how you do this calculation ! Sorry I can't be a bit more helpful.

Thanks mummylin. I've googled a pond volume calculator and estimate ours is around 18,000 litres. Hard to tell as its kidney shaped and guessed the depth.

Use a stick to measure the depth across the entire bottom.

I've sort of vaguely done that tree but the pond is too wide to be able to reach the middle, it's about 7m x 4m. I guessed at a depth!