If you have Word, under the Review tab there's an option to have it read the content using an automated voice. Hearing your content is extremely helpful with editing, like RobertBMarks said. This obviously can't catch pure grammatical issues if you're not already aware of the proper grammar, but it will help with misspelling, awkward phrases, and missing punctuations since it'll highlight the words as it goes for you to see the mistakes.
My other suggestions for pure grammar help are to study the current conventions so you don't need a program, read published books in your genre to see how they do things, and otherwise don't worry too much about needing to match high-level journalism quality of editing. Most people aren't picky about the "proper" number of commas or the like as long as it's not consistently and egregiously neglected.