My husband married me in the year 2015 2nd feb and the next year he married another woman on 21stjan 2016 without my knowledge. We are medical students i am a bhms and he is mbbs and also my cousin. So we were not staying then as we were students. I found out now that he is living with the other girl and when i said to leave me he is not Willing to divorce me. Now what i should do please help.
Due to the lack of information. In your case there are two possibilities as far as I understood -assuming that your marriage was legal and valid according Islam-:
Either you have made it a condition for marriage that your husband leaves you as his lonely wife (and it is written down on 'aqd an-Nikah or witnessed by the witnesses of your nikah): In this case he must abide to it
"The stipulations most entitled to be abided by are those with which you are given the right to enjoy the (women's) private parts (i.e. the stipulations of the marriage contract). (See for example in Sahih al-Bukhari and Sahih Muslim)
In case your husband nevertheless married another woman you have two options taking your 'aqd an-Nikah (or the witnesses which heard the conditions and your husbands agreement) to an islamic court and declare your marriage void as your husband didn't abide to one condition in your 'aqd an-Nikah. This is called faskh الفسخ. Or be patient and stay with this husband and forget about this condition. As Allah says:
... But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah Knows, while you know not. (2:216)
or you didn't make it a condition to marry him and now got jealous. In this case you have two or three options: Being patient and try to get ride of the feeling of jealousy or asking your husband to divorce you if he accepted and gave you all your rights (mahr etc.) everything is fine else you'd have the option to buy yourself free. This is called khul'a الخلع and works as follows you seek a court and tell them or your husband that you can't stay with him any more as you can't stand the fact that he has another woman. In this case you may need to give some or all your mahr back and maybe even buy yourself free and you'd be divorced by court.
Note that a woman can't divorce her husband by talaq الطلاق as talaq is in the hand of the husband based on (first direct sources):
In my opinion the Qur'an addresses the men saying -I found this also as the interpretation of ibn al-'Arabi in tafssir al-Qurtobi-:
O Prophet, when you [Muslims] divorce women, divorce them for [the commencement of] their waiting period and keep count of the waiting period, and fear Allah , your Lord. ... (65:1)
There's a rather da'if hadith saying so:
"... Divorce belongs to the one who takes hold of the calf (i.e., her husband)."
(Sunan ibn Majah -Al-Albani has qualified it as Hassan in irwaa' al-'Alyl (2041) إرواء العليل according to islamqa #26247-)
On the other hand there are many indirect ahadith prohibiting women to ask for divorce which I've already quoted in my answer on How do I ask my husband for khula, when the problem is with me? (for details on the sources refer to that post):
"Women who seek divorce and Khul' are like the female hypocrites." (See for example in Jami' at-Thirmidhi)
"Any woman who asks her husband for a divorce when it is not absolutely necessary, the fragrance of Paradise will be forbidden to her" (See for example in Jami' at-Thirmidhi)