In order to fish in The Sims 4, you will need to go to a lake or a river and look for a sign post. Interact with this and select "Fish" to begin. This will make your sim fish, and eventually catch fish that will be added to your inventory. You will get skill points in fishing as you do this, and like other skills you go from level 1 (worst) to level 10 (best). The higher your skill is, the more easily you can catch fish, as well as being able to catch rarer and bigger species of fish.
How to fish 20 different species of fish for the Angler aspirationThe most obvious solution is to fish in lots of different locations. Travel to both commercial and non-commercial areas of all the cities you have unlocked (which are 3 if you don't have any expansion packs). Most fish can be caught anywhere, but some of the rare fish can only be found in certain areas. So make sure to spend time in both rivers, lakes, as well as the small ponds in the Desert Oasis park.
In addition, always fish with a bait. Using a large or a medium fish as a bait will give you a much higher chance of fishing up those rare fish that don't really show up very often at all. Keep in mind that the bait will disappear after only a few minutes, and in order to get a new bait, you will need to cancel the fishing activity, then reselect it with a new bait. This is a bit of a hassle, but it's worth to always fish with a bait, because it really makes it much easier to catch rare fish.
Even when you fish in different spots with bait, it might take you a very long time to find 20 different species if you are a bit unlucky. Luckily for us there is another method to getting the angler aspiration done that requires way less luck, so keep reading to find out.