How To Set Default Page In Asp.Net
Here we consider two cases.
1. When the project is run from Visual Studio.
2. When the project is run from IIS.
Lets consider them in detail.
If you are running a project using Visual Studio, this can be done as follows.
Right Click on your desired Page and Select Set As Start Page.

But what if for Websites that are hosted on IIS.
If there is Default.aspx, it would render first.
If we need any other page instead of Default.aspx, we can follow below method.
For that the best solution is to add following code on Web.Config.
<system.webServer>
<defaultDocument>
<files>
<clear />
<add value="MyPage.aspx" />
</files>
</defaultDocument>
</system.webServer>
The above code needs to be placed inside configuration node.
In the place of Mypage.aspx, you can add your desired page.