Sentences must begin with a capital letter.

Literal

Sentences [topic-は] capital-letter [means-で] write-begin must.

書き始める is a compound verb: 書く (write) + 始める (begin) — 'to begin writing.' ~なくてはならない is a variant of ~なければならない, both meaning 'must.' This sentence is about writing conventions, not Japanese-specific grammar.