How do I download apps for my windows Phone?

[Diëgo]

When you start your windows phone first time, you'll get the possibility to add accounts, like Windows Live, Facebook, Twitter,..
When you start up the Marketplace, your phone will ask you to register your Windows ID.
Once you made that, you can download apps from the Marketplace at your Phone.
It's really easy, Windows Phone is designed very well.

I hope I did help you and did understand your question correctly !