Open Modal dialog in SharePoint

You can use a JavaScript function for this purpose.

1) When you don't require a return from the Modal dialog.

    function openModalWindow(strPageURL) {
        var options = {
            url: strPageURL,
            title: "Title goes here",
            showClose: true,
            width: 600,
            height: 400,
            allowMaximize: false
        };

        SP.SOD.execute('sp.ui.dialog.js', 'SP.UI.ModalDialog.showModalDialog', options);
    }

2) When a return value is required to be captured by the Parent page

Include a callback parameter in the options specified

Option 1: Include in the options parameter collection

function openModalWindow(strPageURL) {
        var options = {
            url: strPageURL,
            title: "Title goes here",
            showClose: true,
            width: 600,
            height: 400,
            allowMaximize: false
            dialogReturnValueCallback: CloseDialogCallback
        };

        SP.SOD.execute('sp.ui.dialog.js', 'SP.UI.ModalDialog.showModalDialog', options);
    }

Option 2: Place the code after other option parameters are defined

var options = {
                url: strPageURL,
                allowMaximize: false
                showClose: true
            };
            options.dialogReturnValueCallback = Function.createDelegate(null, CloseDialogCallback);

            SP.SOD.execute('sp.ui.dialog.js', 'SP.UI.ModalDialog.showModalDialog', options);


In the invoked function, you could specify the actions to do once the Modal Dialog closes, such as refreshing the parent page

function CloseDialogCallback(dialogResult, returnValue) {
            if (returnValue.d) {
               /*Do something */
            }
            if (dialogResult == SP.UI.DialogResult.OK) {
                SP.SOD.execute('sp.ui.dialog.js', 'SP.UI.ModalDialog.RefreshPage', SP.UI.DialogResult.OK);
            }
        }

3) Closing the Modal Dialog (Add script to the page in the Modal Dialog)

This can be done in two ways

• Without referencing the SP.UI.Dialog.js

function closeOnCancel() {
            window.frameElement.commonModalDialogClose(0 /* 0 for cancel */, 'Cancelled');
        }

function closeOnOk() {
            window.frameElement.commonModalDialogClose(1 /* 1 for ok */, 'OK result');
        }

- OR - 

• using the SP.UI.Dialog.js

Reference the JavaScript on your page.
<script src="/_layouts/15/SP.UI.Dialog.js" type="text/javascript"></script>

function closeOnCancel() {
            SP.UI.ModalDialog.commonModalDialogClose(SP.UI.DialogResult.Cancel, 'Cancelled');
        }
function closeOnOk() {
            SP.UI.ModalDialog.commonModalDialogClose(SP.UI.DialogResult.OK, 'OK result');
        }

To call these JavaScript functions from server side code use ScriptManager:

ScriptManager.RegisterStartupScript(this, this.GetType(), "closeScript", "closeOnOk();", true);

However, the final JavaScript function might give out a JavaScript error in some Internet Explorer versions.