How To Induce Periods?

Hi,great day:) .My last period is on November 18,2014. Im expecting my menstruation last December 18-24 but until now I dont have my period.I had a boyfriend and we dont use any contraceptive or even withdrawal.And my period goes normal last few months.I took pregnancy test,a good brand because I want the answer to be accurate. It shows negative result.What should I do to have my period? Please help me .thank you.