Since you're aiming to improve your writing and not just make money (awesome!) then I would recommend doing a search in Hubpages for articles that offer advice on writing, both technical details (such as grammar) and ways to find inspiration. Read the ones you think might be helpful -- reading and writing go together, after all, and one can improve the other -- and then write thoughtful comments at the bottom.
Also look for hubs that offer writing prompts/ideas. Browse the forums to see other people talking about what writing issues they struggle with. Sometimes we're not even aware of what needs improving or why we are having trouble until we do some self-analysis or let others (constructively) criticize us.
And then of course that whole "practice, practice, practice" thing is absolutely true. Hubpages as a platform offers you advice, such as ways to better engage an audience or the ideal length of an article. You don't have to immediately make public anything you write here. You can save it unpublished and make changes or additions as you learn and grow, so that when the hub is ready it will be your best.