The French question words
Every question French can ask starts from a closed set of eight words. Learn them as a set and the asking half of conversation is solved; leave them scattered and you will reach for the wrong one for years. Here is the full inventory, then the drills.
The invariable seven
asks about people: Qui parle ? (Who is speaking?), Avec qui est-ce que tu parles ? (Who are you talking to?). It never changes form.
asks about things: Que fais-tu ? (What are you doing?). Before a vowel it elides to qu' (Qu'est-ce que c'est ?). When the word stands alone or follows a preposition, it swaps to : Quoi ? (What?), À quoi tu penses ? (What are you thinking about?). Same meaning, different position.
asks about place: Où es-tu ? (Where are you?). The accent on the u is not decoration - it is the whole difference between où (where) and ou (or).
asks about time: Quand est-ce que tu pars ? (When are you leaving?).
Comment asks about manner, and doubles as the how-are-you word: Comment ça va ? It is also how you ask for a repeat - Comment ? is the polite version of Quoi ?
asks for a reason, and the answer it expects begins parce que: Pourquoi est-ce que tu ne viens pas ? - Parce que je travaille.
asks about quantity. On its own it means how much (C'est combien ?); before a noun it takes de, with no article: Combien d'enfants avez-vous ? - never combien des enfants.
Quel: the one that agrees, and est-ce que
The eighth member behaves like an adjective, not a pronoun. (which, what) sits next to a noun and copies its gender and number: quel jour (masculine singular), quelle heure (feminine singular), quels amis (masculine plural), quelles choses (feminine plural). All four forms are pronounced identically, so the agreement is a writing skill: Quelle heure est-il ? but Quel temps fait-il ?, and the exclamation Quelle nuit ! (What a night!). Any question word, quel included, can be followed by - , and this is the pattern worth over-learning as a beginner: Quand est-ce que tu pars ?, Pourquoi est-ce que tu ne viens pas ?, De quelle femme est-ce que tu parles ? Est-ce que asks nothing and means nothing by itself - it is a fixed hinge that turns a statement into a question while leaving the word order untouched, which makes it the one pattern you cannot get wrong. Inversion (Pars-tu ?, Que fais-tu ?) is the tighter, more written alternative; plain rising intonation (Tu pars quand ?) is the everyday spoken one. All three are correct French.
The qui / que trap
The pair learners confuse longest is qui and que, because English "who" and "what" feel further apart than the French forms look. The rule in direct questions is clean: person means qui, thing means que. Qui cherches-tu ? expects a person (Ma mère); Que veux-tu faire ? expects an activity. The drill below sets them side by side until the choice is automatic.
For the follow-on system - quel versus lequel, and the contractions auquel and duquel - see the quel and lequel article. The three question patterns get a fuller treatment in the question words article.