hi,
i'm guessing you have a tank around 30 gallon or so.  You will need either an external power filter that hangs on the side of the aquarium or a canister filter. The external canister filter is the next step up in power filters and is designed to filter the water for most dirtiest aquatic pets,  such as water turtles etc. Canister filters are also more expensive.

You can go with either the external power filter or the canister filter.  These filters should be available at most reputable pet stores. So if you have a 30 gallon aquarium , buy a filter that  is rated for at least 40 gallon aquarium or more.