Legal Marriage

This is udaykiran,22yrs and i am loving one girl named, sushma,she is inter 2nd year and just 16year, we are trying to get marry by 2019 so she would be 17yrs by next year (that is 2019)....i think it is impossible to get marriage certificate...so shall i seek any legal agreement to marry her for future until she turns 18??? Because their parents are forcing her to marry by next year that is 2019...they already know she is in love with me and never wants to study....please help me to get legal agreement to get marry her for future...