Here are a few websites that could help you on your way. Some of them are writing based, others more for marketers, but all are useful resources and worth exploring.
40 Content Writing Tips to Make You A Better Marketer Now from CoSchedule - skip the top part about resources and go straight to their 40 tips. There is some great advice about outlining, headlines, deadlines, and researching.
Writing Clear, Concise Sentences from The Writing Center at UW–M