The size of the water change will depend on the size of the tank and what you are trying to accomplish with the water change. You want to have ammonia levels as low as possible and at all costs keep it under 1 ppm. If your tap water has 0 ppm (not all tap water is 0 ppm) of ammonia and you do a 50% water change, you cut the current measurement in the aquarium in half. Your .25 ppm reading becomes .125 ppm. Personally, I do 50% water changes because it is easy to calculate the end results.
For the pH, you should know that it will change through out the day. The pH change is gradual and as long as it does not drop out of their acceptable range they will be fine . You will want to measure the tank at the same time of day to get a comparison of a change over a period of time. It is also natural that the water will become more acidic over time; Nitrification, changing of ammonia to nitrite, will acidify the water.
Try to get some live plants in the tank as this will reduce the ammonia waste in your water. Also, shoot for changing 100% of the water per month in at least 2 water changes.