Good free online resources to improve your writing

This is a little embarrassing to admit, but I never had a good education and I really don't have the best writing skills. It's not a pleasant experience to constantly worry about, for example, whether I placed my comma correctly. I think I might be able to benefit from something like an online writing tutorial. I can find many websites on writing with a quick Google search but problem is, as with anything you find online, I can't tell the good ones from the junk.

I think my biggest problems is punctuation: where exactly to place periods ("full stop" in other countries), commas, colons, and semi-colons; and identifying sentence fractures and run on sentences. I also don't really understand what a comma splice is.

Can anybody recommend any English writing resources?