Sentences always begin with a capital letter, but some words need a capital letter even if they are in the middle of a sentence.
In Key Stage One you probably learnt to use capital letters for these words.
1) People's names
2) Place names (towns, countries, streets, schools, shops etc.)
Woodlands Primary School
Marks and Spencer (little words like and don't usually have a capital letter)
3) Days of the week, months of the year and special dates
4) The names of books, films and TV programmes
Harry Potter and the Chamber of Secrets
The Hundred and One Dalmatians