I want to get files on C# downloaded from computer to default folders Document or Music on windows phone 8. How can i get them, if it possible?
Those two folders are available to a developer, as described in the answer on Windows Phone 8: Media file access
The api for songs and pictures seems to be MediaLibrary according to the "Data for Windows Phone" page on MSDN.
It allows to add (SaveSong, SavePicture) and Delete songs and pictures from the library.
To be able to do this, you need the Capability ID_CAP_MEDIALIB_AUDIO and ID_CAP_MEDIALIB_PHOTO set in the app's manifest.
The documents folder can be accessed through the StorageFolder Api as described in the File access sample
StorageFolder storageFolder = KnownFolders.DocumentsLibrary;
To download files from a computer is an entire different question and requires some more information from your end:
If you are going to write a program for that computer to share files to your phone, then these Windows Phone 8 Networking Samples can provide some information on how to get sockets, nfc, bluetooth or http going on windows phone 8.