How many gallons is your tank? You need to purchase a filter for your tank. You cannot cut corners. Fish keeping is somewhat expensive up front when considering equipment; filters, filter media, air pumps or powerheads, ornaments, fish, varied fish food, etc.
To me it sounds like you've become bored of late and have a wild idea to keep fish because your buddy has a tank, God forbid. Is this something you can afford the time and expense on? Why not think about it for a week or two? Perhaps this urge will pass and you'll come to realize it was indeed a crazy idea to begin with.