If you want to transfer your favorite ringtone from your computer to Google Nexus One phone then its easy to do. Ringtone that you want to copy to Nexus One should be one of the following fomats:

Google Nexus One supports the following ringtone file formats MP3, WAV, WMA (8), MIDI, M4A, AMR, OGG Vorbis. If you want to create a custom ringtone for Android phone check this: how to create custom ringtone

How to transfer & apply ringtone to nexus one

Step-1

Connect Nexus One to computer

1. Connect your phone with computer using USB data cable that comes with your phone.
2. Tap and drag down the notification bar from the top of Nexus One screen.
3. Tap “USB Connected”.
4. Tap “Mount” button.

Step-2

Transfer ringtone file to Nexus One

1. Open My Computer on PC, you will see a new drive icon under “Devices with Removable Storage” section, open this drive.
2. Open the ringtone folder and copy your favorite ringtone file from computer to that folder. (see also: how to create ringtone)
3. Again tap and drag down the notification bar from the top of Nexus One screen.
4. Tap “Turn off USB storage”
5. Tap “Turn off” button

Step-3

Set Nexus One ringtone

1. Run “Music” application from Nexus One (You can type “Music” in the search box at the top to find it)
2. Tap Songs
3. Locate your ringtone file that you have copied.
4. Tap and hold on the file this will open options menu, select “Use as phone ringtone” from options