you will need coconut milk! I would fry the garlic for a few minutes then add green curry paste and fry lightly for about 10 minutes, do not burn, then add coconut milk, 1 can for 3 people plus half a can of water, then add fish, potatoes, onion, chillies and a couple of splashes of fish sauce, simmer on a very low heat for a couple of hours, covered, then add tomatoes, red peppers and cook again on low heat, taste it and add more fish sauce, lime juice etc to suit, you could even add a fish stock cube at the start for more flavour. As with any recipe, its trial and error, keep tasting it and add what you think is good. You may wish to add the fish later as it will only take 15 minutes to cook through, but i like to add it early and cook on very low heat, it adds more flavour. good luck.