That is an extremely open-ended question. My advice would be to do some research in both and find out which one goes in the direction you want to go. PHP is usually the cheaper option to host because it runs on Linux. However, ASP.NET is usually the better choice in the enterprise because of its ties into all things Microsoft (SQL, WCF, desktop code, etc.).
Here is a place to get started with each:
PHP:
http://www.w3schools.com/php/default.asp[
^]
ASP.NET:
http://www.w3schools.com/aspnet/default.asp[
^]
Learn about both. It will give you a good base about what is out there. From there, you can make a more-informed decision as to which way you want to proceed. Once you know that, start reading tutorials and watching training videos until you have a very solid knowledge of how to do programming in the language of your choice. From there, start doing it. Hands-on is usually the best teacher, but you are going to need to do a lot of studying first. Otherwise you will be frustrated because even simple things will be tough.