Relative clauses in Dutch begin with a relative pronoun and follow the noun they describe. The relative pronoun is die for de-words and all plural nouns, and dat for het-words: de man die roept, het kind dat huilt, de kinderen die spelen. Since most Dutch nouns are de-words, die is by far the more common relative pronoun. A useful check: if you can replace the pronoun with hij or zij, use die; if het, use dat.
When the relative clause involves a preposition plus pronoun referring to a thing rather than a person, Dutch uses a waar-compound instead of a preposition plus die or dat. Rather than de stad in die ik woon, Dutch says de stad waarin ik woon. Common waar-compounds include waarover (about which), waarmee (with which), waarvoor (for which) and waaraan (to which). For people, the preposition plus wie is used: de persoon met wie ik werk.
In informal spoken Dutch, relative clauses are often simplified: de man waar ik mee werk instead of the formal de man met wie ik werk. Recognising both registers is important for comprehension. In writing and formal speech, use the standard forms. Remember that relative clauses send the verb to the end because they are subordinate clauses, so all verbs pile up at the end: het boek dat ik gisteren heb gekocht.