Read, Read and Read
There is honestly no better way to develop your language skills than to increase your exposure to words, sentences and content. Read lots and all that reading will get paid off in being a successful freelancer despite not having a formal degree in language or writing.
Listen to Music, but Only the Right Kinds
Listening to music has developed my language skills to an extent, as the poetic language used in songs usually means you'll encounter words you don't come across in everyday conversation. This adds to your vocabulary making you a better writer as you will have more words to use in your writing. However, be sure to select the songs you listen to carefully, as most rap and hip-hop based songs use informal language that might not be suitable for article writing or formal writing.
The Dictionary is Thy Friend
Have a dictionary around at all times so you can look up a word while reading the newspaper, or when you hear a word you don't know the meaning of in a song. Trust me this helps!
Practice Makes Perfect
The only way to be an excellent writer is to write often and give your pieces to a language professional to proofread. They will correct your language-based mistakes, and perhaps give a comment or two on your style of writing based on the topic being written about. Write to newspapers and magazines whenever you can. You never know, you may see a piece of yours in tomorrow's paper!