I need to open a modal dialog pop up box when i click a button on the edit form but at the same time i need to save the data on the edit form. Example: I have a SPList say Report. In the edit form of this List, I have a button, the onclick function of this button opens a modal dialog pop up, i need to save the list item before (or) during the time when modal dialog pop up opens up.
Yes it is possible. But when you save the form, page will postback and reload, meaning your pop up will go away.
It is called "PreSaveItem()"
Just open your pop-up in that method and return true.
for opening a modal dialogue box you can make use of dialog widget from jQuery UI library.
I am sure there are multiple other samples in the internet in case you do not want to use jquery,