qui, que, où, dont

Relative pronouns join two sentences about the same noun. Qui is the subject of the verb that follows: l'homme qui parle. Que is its object: le livre que je lis. Que becomes qu' before a vowel.

Où is for a place or a time: la ville où j'habite, le jour où il est parti. Dont replaces a phrase with de: le film dont je parle, because you say parler de.

A quick check: if a verb comes right after, it is often qui; if a subject comes right after, it is que: la fille qui chante, la fille que tu connais.