Join two ideas: the book that I read, the city where I live
Que covers English that, which and who. Unlike English, it can never be left out.
El libro que leí es genial. (The book [that] I read is great.)
La chica que vive aquí es médica. (The girl who lives here is a doctor.)
English drops "that" all the time — Spanish never does.
Donde (no accent here) links to a place.
La ciudad donde vivo es pequeña. (The city where I live is small.)
Ese es el restaurante donde comimos. (That is the restaurant where we ate.)
Quien/quienes refers to people, mainly after prepositions (con, a, para...). In everyday speech, que is fine when there is no preposition.
La amiga con quien viajé es de Cádiz. (The friend with whom I travelled is from Cádiz.)
El chico que conocí ayer... (The boy I met yesterday...) — que, no preposition
Lo que means "what" in the sense of "the thing that".
No entiendo lo que dices. (I do not understand what you are saying.)
Lo que más me gusta es el queso. (What I like most is the cheese.)
Traps for English speakers
These are the errors English speakers make most often.