There is now a wealth of information and help on the web to help you with grammar, spelling and other language issues.
I am continually looking around for the best sites. So far, these are my top choices, in approximately my order of preference. Everything is free.
Many colleges in the United States have online writing labs or OWLs. This one is extraordinary: comprehensive, well informed and easy to use.
A free online version of the dictionary I keep on my desk.
Ask Oxford. Tips on writing, language and spelling; games; links to other sites. You can even have the word of the day sent to you by email.
Useful advice on grammar and writing styles, in the context of essay writing. Set up by the University of Otago in New Zealand.
Lots of useful English language resources, many of them online and free.
Huge site with lots of useful information and links.
An academic site that allows you to explore how words and phrases are used in context. Rather complicated to use, but very useful if you want to delve deeper into language use.