Fill your aquarium with tap water , if you're using pail to transfer the water make sure the pail chemical free.....example detergent ........i recommend buy a new pail specifically for aquarium use no sharing with others to make sure no cross contamination.
You may also add in PSB bacteria to help start your filter system and make your water more lively.
At this point ,if you like to decorate your aquarium you may.
Let your filter system to set........ say for about 6 -8 hours or until your aquarium water is crystal clear .
Now is time to go to your favorite aquarium shop to stock up the fish.
You should also stock up some medication just in case you needed it . Normally one's will keep Anti -Itch , Anti-parasite and general aid medication and there are hundred of brand in the market and all serve the same purposes .
TIP : before you run down to the aquarium shop plan out what type of fish you want to keep :-
* community type which is there's are several species co exist in your tank
* carnivore type which only one type and some carnivore will eat or fight with each other so the best is to keep one