How can I improve my English spelling and grammar?

A moment ago I had a question moderated because it had too many typos. This is no big deal, but it does serve as a reminder that my english spelling and grammer is not quite up to scratch.

I live in spain, on a small island called Mallorca where there are no adult schools in English or anything like that.

People tell me that my English spelling is not that bad, or that they can’t tell that I’m not formally educated in english. But I do often have people point out spelling mistakes I have made.

Seeing as I am interested in writing a book, I think it is vital that I improve my English.

How could I, a 27 year old who lives in Spain improve my english? are there any sites that help with this kind of thing? or online courses?

The way I write in English, is I have more or less memorized how to spell every single word, I don’t know any of the rules, such as what letter before what letters except in the case of what exception. Is there anywhere I can learn these rules?

Thanks.