文は大文字で始まる。

English Translation

Sentences begin with a capital letter.

Is the correct pronunciation おおもじ or だいもんじ?

@cstuartroe
おおもじ is the only correct pronunciation in this context.
だいもんじ spelled as 大文字 usually refers to 大文字焼き, a traditional event in Japan. You can check out the pictures on Wikipedia.