I have to restrict user from entering/selecting date less than current date

Date field automatically takes some random year value while entering date value manually.For example: while entering date as 18/03 /2 after typing 2 in year it takes some random year as 2002.

The most straight forward means of doing this will be by adding validation rules on the field in salesforce.  Of course that is a server side solution and will not prevent users from seeing the old date.  

Our date picker uses the JQueryUI DatePicker,  which has a substantial number of customization possibilities.  You can create a custom renderer that leverages the “minDate” function of the date picker.  See more documentation here

Thank you Rob, that works.
one more question, I want to restrict manual entry like typing a date value in the date field. how to restrict that here ?.
 In jquery I used to make input field readonly thats works for that,but here making field readonly from standard functionality doesn’t serve my purpose.