how to call one js file in entrie project?

Question

hello friends
i have a project. where i have several form in that.
using master page and each content page have form, now i want to check client side that user cannot leave blank nd cannot give wrong values in form. so i create functions , file main is validate.js. here i create funcations of each form, now i want to call it on content page, how to attached this file so that i can all desired function of particuler form. please tell me where and how to attached that js file and how call functions on button event
thanks