Sharepoint: New Form, Edit Form Modifications

Goto the Newitem page of the list
Start edit pageIf there is no edit page available then past the next code in the url of your browserjavascript:MSOTlPn_ShowToolPane('2');Now goto the edit page option again
Add an content editor webpart.At the bottom of the property box you can add a link to an JS file inside an Documentlibrary or an location on system.
Now you can edit the javascript below.

Using Javascript to Manipulate a List Form Field