I want to marry legally!

Sir, i need help to know about this issue? I am 19years boy, I want to marry a girl how's age is 18years old, i am having my parents in our support for this decision basically her's parents are not having any sort of information that i am having any affair with her so we don't want tell them that we are going to marry each other. Bt the problem as we know that the proper age of getting married is 21 for boy and 18 for girl. In this case can we get married?? ?? As i know many complications are going to be happen if her'parents get to know that we tried to get married.they can report a FIR against me as kidnapping, harassment etc. but i just want to get married with in under our Indian constitution in a safety way for my family nd our relationship too and i heard many of these type of cases that police complications for the boy's family that they tried to get marry in a forced to the girl for the marriage. If i am having any written legal document regarding to that i am not forcing the girl to getting married with me and she is agree with this marriage and happily accepted by my parents sighed by the girl!! after that marriage is possible or not? If it is yes after marriage we don't want to tell them that we are married already until some other person arrived to talk about her marriage to her's parents. Then we will tell them that we are already married. Is it possible?? To get married legally right now in this age? if yes So, how we have to approach it? ANd what legal documents are needed for this and other requirements?