If you’re preparing for a French test, you should focus mostly on the second category but if you’re aiming for a C1 or C2 level you should also get some exposure with more formal language.
Best resources for formal language
You can read newspapers like Le Monde, Le Devoir (Canada), Courrier International (translation in French of articles from all over the world).
Read classic French litterature and some of the contemporary writers (André Malraux, Antoine de Saint-Exupéry, Albert Camus, Georges Simenon, Amélie Nothomb, Jean-Christophe Ruffin, Delphine de Vigan…).
Listen to France Culture, Radio France International, go to conferences if you live in a francophone country.
Best resources for standard to casual language
Read blogs about topics you like and some contemporary novels (some books by Albert Camus have simpler language, Anna Gavalda, Fred Vargas, Leïla Slimani, Gaël Faye, Guillaume Musso, Marc Lévy, Maxime Chattham, Michel Bussi…).
Read online newspapers like FranceTVinfo, 20minutes.fr, wedemain.fr and watch TV5Monde and French (or francophone) TV in general.
Listen to podcasts, TedTalks and some Youtubers (in their mid-thirties, for example Norman, Natoo and Paul Taylor, or over if possible because there’s clearly been a shift in the language in the last decade).
Watch movies and series on Netflix but select the ones adressed to kids or to an older audience (35 and over). One of my favourite series in that category is “Call my agent” (“10%-Dix pourcents” in French) because it’s casual workplace French, the dialogues are very realistic and actors are fantastic.
Best resources for very casual language and recent slang
I’m not an expert in that one (not my generation!) but here are a few suggestions to start with.
Dictionnaries
Le dictionnaire de la zone
TV5 Monde : “Le parler jeune pour les moins jeunes“
French rap and RnB
Maître Gims
Soprano
Aya Nakamura
Youtube channels
Streetfrench.org for explanations about the slang
Youtube channels made by people in their twenties or younger
French reality TV