There are three instances when capital letters must be used:
For the first letter of the first word in the sentence, e.g. The crocodile is hungry.
For ‘I’, the first person singular pronoun, e.g. My brother and I get on terribly.
For proper nouns, e.g. My pet crocodile is called Harry. There are eight categories of proper nouns:
A particular person or named object, e.g. Mr Gale, the Batmobile
A particular place e.g. Mairangi Bay
Organisation, company, brand e.g. Rangitoto College, Apple, Under Armour
Text titles e.g. Better Now, Avengers: Infinity War, Bridge of Clay
Day, month, particular named holiday e.g. Monday, May, Mother’s Day
Specific events and time periods e.g. World War I, the Middle Ages, the Millenium
Country, nationality, language e.g. China, Chinese, Mandarin
A religion or deity e.g. Islam, God