Very general question.
you can do it with asp.net and use WCF if you want to. That is one way to have a site that is platform independant (provided you sort out the styling) as it is not hosted ont he device consuming it.
If you want to write an app directly for windows phone 7 ... then you probably want to use google ;) you can use Silverlight to make the UI adn things like transitions and general page layout is snazzy as (insert you own expletive) - and amusingly easy.
If you need a database that many users connect to or have a reason for centralising something and need communication over internet then WCF comes in very very handy. However you do not NEED it in all cases.
Windows phone 7 has some simple local storage mechanisms ... in fact i haven't written a WP7 app yet that really needs a WCF service behind it (and i'm a WCF junky).
HOwever i do not know of a way to go from windows forms -> wp7 directly. I imagine several people will have some software you can buy to do it though as there are always many people working on projects liek that.
If you want the simple route ... make an asp.net webpage and design it for WP7.
However if you want something that has the WP7 experience then you want to play with WPF - it is fairly easy ... especially if you can play with expression blend. Expression blend is the tool that makes WPF/silverlight a lot of fun imo.