。
I resolved to study harder from now on.
Literal
Hereafter more diligently study [quotative-と] resolved.
今後 (こんご, 'from now on, hereafter') is a formal time expression. 決意する (けつい, 'to resolve, determine') is more serious than 決める ('to decide'). と quotes the content of the resolution. Subject omitted — the speaker is understood.