The essential part of an Islamic fast is to not eat, drink or satisfy sexual desire during the day; with the conscious intention of observing a fast.
The day starts at astronomical dawn and ends at sunset. You must stop eating at dawn and not eat again till sunset.
Its sunnah to eat a little just before the start of the fast (called Suhoor ). The fast is concluded (called Iftar) with a meal or water at sunset. You can eat, drink etc. through the night.
وكلوا واشربوا حتى يتبين لكم الخيط الأبيض من الخيط الأسود من الفجر ثم أتموا الصيام إلى الليل
And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the night (sunset)
The exact starting and ending times vary depending on your location and time of the year, you may use a website where you type in your location and it calculates the time for you, e.g:
- http://www.islamicity.org/PrayerTimes/ (use starting time of Fajr and the time for Maghrib)
- https://www.islamicfinder.org/ (same as above)
- https://www.timeanddate.com/sun/ (use starting time for astronomical twilight and the time of sunset)
You may also find apps for location aware devices which do the same thing.