I'm assuming 15,700 is your gross annual salary. Since there are 12 months in a year, your monthly gross salary (discounting variations in the number of days in particular months) will simply be 1/12 of that.
15,700 / 12 ≈ 1308.33
If you are trying to figure out your net take home pay (after deductions for benefits, FICA, federal and state taxes, etc.) that's a more complicated question and will depend on a lot of factors such as how many exemptions you claim on your W-4, what you pay for medical, dental, 401(k), etc.
So the short answer is all we can tell you is your monthly gross pay and can't say much about your monthly net pay.