Aquarium water quality can be a difficult topic especially for people new to fish keeping. pH measures the water's acidity/alkalinity. The pH scale runs from 0 (very acidic) to 14 (very alkaline) and pH7 is an ideal level for the majority of fish (and also humans as our blood pH is around 7.3). Some fish do prefer different pH levels so make sure you check this before you buy.
A temperature of 24-26 degrees is standard for a tropical aquarium and is regulated by a thermostat in the heater.
A thermometer should be visible in the tank to ensure the temperature does not fluctuate. Ammonia and nitrite levels should be regulated and although they both occur naturally from fish waste, bacteria living inside your filter convert these dangerous chemicals intro nitrates which is less harmful.
Regular water changes should keep these levels close to zero. However, bacteria take time to build up and new tanks are particularly susceptible to high levels of dangerous chemicals. A filter boost can be added to promote the growth of bacteria and speed up the process.
To maintain ideal water conditions and prevent the build-up of dangerous chemicals in the new aquariums, perform water changes at least once per week removing 20% of the tank's volume and replacing it with clean and dechlorinated water. You can reduce the frequency of water changes to once fortnightly when the chemical levels have settled down. You will need to rinse the filter media on a regular basis to remove large sediment and the impellor inside the filter from sludge, but in both cases you must ensure you rinse with water from your tank and not from the tap.