Improvement of language comes from reading. When you read extensively, you familiarize yourself with word usage. For example there is a difference between 'week' and 'weak'. Weak means not strong and week means a week as in Monday to Sunday.
By reading books, start by just reading easy and elementary ones, you would be able to differentiate between 'there' and 'their' where 'there' is related to a place as in 'I have been standing there all the time.' as opposed to 'their' as in 'Their uncle's name is Joe.'
There are plenty of information available on the Net where you could find interesting sites that teach you word usage as well as grammar.
However, there is nothing better than reading and making use of a dictionary occasionally. I am pretty sure you would progress faster this way.