Asp.Net
Write following code to Redirect to Next Page from Login Page.
Response.Redirect("NextPage.aspx");
Here
NextPage.aspx
Should be in the same project folder.
Otherwise, you need to provide the relative path of the page.
Windows Form
If you are asking about Windows Forms, then do like below.
First you need to create a object of the Form and then show it.
NextForm form = new NextForm();
form.Show():