The journal evaluation process is time-consuming. Bigger journals in particular, receive a huge volume of submissions and managing them takes a lot of time. It is quite normal for a paper to remain "With Editor" for a few weeks before the status changes. It is too soon to write to the journal. You would not want to give off the impression that you are impatient or overly pushy. It's best to wait for at least two more weeks before you send an email to the journal editor. Please be polite and courteous in your email, and position it as a request for an update rather than a reminder. 

You might want to take a look at our handbook of templates for communicating with the journal.