There are several Writing Centers (similar to our Reading and Writing Studios) at different universities that have put together some outstanding resources on general writing issues and concerns. Below is a list of some of the sites I have found to be the most helpful.
If you need help with grammar and mechanics, I highly recommend the grammar course on Khan Academy: https://www.khanacademy.org/humanities/grammar. It will lead you through learning grammar on an interactive online website.
On these sites, you will find resources on writing topics such as:
Grammar and Mechanics
Commas
Fragments and Run-ons
Editing and Proofreading
General writing topics
Flow
Writing introductions
Writing conclusions
Outlining
Transitions
Organizing Your Writing
Revising drafts
Formatting your paper
APA citations and formatting
MLA citations and formatting
Revising Your Papers and Revision Techniques