I think as a new writer starting out, you charge per amount of words you write (minimum I've seen is charge per 500 words). As you then become more skilled, your rates increase.
One thing I'll suggest though is that you write about 10-20 articles in various niches that you can use as samples to clients.
One thing I'll suggest though is that you write about 10-20 articles in various niches that you can use as samples to clients.