Hi Jack, welcome to the forum!
To keep tropical fish, you need a tank (as big as you can afford; it'll be easier in the long run!), heater, filter (again, as big as you can afford), lights and some test kits and dechlorinater.
All fish produce ammonia as a waste product; this builds up in the water and will eventually kill the fish, but luckily for us there are bacteria who 'eat' ammonia and turn it to nitrite and then into nitrate which are much less toxic. You remove the nitrates with water changes.
'Cycling' a filter just means that you feed your filter with ammonia and let the bacteria grow
before you add any fish to the tank. This is much safer and easier than doing a 'fish-in' cycle (by using the fish to produce ammonia) you risk killing the fish if you get it wrong and it also involves doing huge (sometimes 90%!) water changes. We
always recommend fishless cycles on here
There's more information on cycling here in the Beginners Resource Centre http
/www.fishforums.net/index.php?/topic/277264-beginners-resource-center/
I agree with AstonN24vantage; Piranha sound cool, but they're really not a great fish to keep; they're actually really shy and need to be in groups, but then, in tanks, they can't get away from each other and the smaller ones get eaten...
Have a look round your Local Fish Shop and see what you like the look of and we can advise you more.
Hope this helps
