You've got the tool to do this online here, but heres the normal PC way..

What you need:
1. A Mobile phone that supports MP3 ringtones
2. Audacity
3. LAME
4. dBPpoweramp Music Converter (dMC)
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1. Install LAME
Contrary the recursive acronym, LAME is an open source MPEG-1 audio layer 3 (MP3) encoder. It’s a great encoder that is both free as in speech and free as in beer.
Log in to see links
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2. Install Audacity
(Log in to see links)
Audacity is a free audio recording/editing/mixing application that runs on just about every major OS. The installation is nice and easy. Just grab the Windows installer and run it.

After you have Audacity installed, go to File > Preferences > File Formats and click the Find Library button in the MP3 Export Setup section at the bottom.
Browse to the location of the lame_enc.dll file (from Step 2 above) and select it.
You should be all set to encode/re-encode MP3s now.
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3. Install dBpowerAMP Music Converter
Download and install dMC on your PC - the software lets to inter-convert between loads of music formats, you can get the latest version from Log in to see links - but i would recommend an older relese (r10 or r9) because from r11, the MP3 encoder in on trial basis
Release 10: Log in to see links
Release 9: Log in to see links

As far as i can remember all the releases have the Wave and MP3 encoders - so you probably don't need to download anthing more now.

Note: During install, make sure, you select 'Convert To' on Explorer Right Click in Windows Shell Integration (you'll be prompted after install)
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3. Trim MP3 down to size
In most cases you only need about 30 seconds of audio for a decent ringtone. Anything beyond that is just going to waste precious memory on your phone. To crop the song, we will use Audacity.

Open the MP3 you want to use in Audacity and use the cursor to select about 30 seconds of the audio

Then Edit > Trim (Control-T) and it will cut off everything that isn’t selected. For more precise trimming, you can now zoom in again and repeat the process.

Next hit File > Export as MP3 and save the new file.
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4. Reduce file sizes
Now your ringtone is ready - but the disk space is way too large for a ringtone - time for dBpower music converter to step in - right click on your ringtone and choose 'Convert To' > MP3 (Lame)
Reduce the Constant Bitrate to 80 or 96 (even 64 will do - below that the quality would be awful)
Choose the output folder and click the Convert button - the once completed, your ringtone is ready to use.
Unchecking Preserve ID Tags berfore converting - will remove info such as Title, Artist, etc from the MP3 file and reduce the size very slightly - but wont make a major impact.

Now that you know how to do all the basic stuff, you can do some experimentation with Effects and mixing in with Audacity

You can use dBpowerAMP to convert your music to MP3 or other formats - get all the codecs from here (pre-r12):
Log in to see links

Now you can use your ringtones and share it with the world!
http://www.mobiles24.com/do-upload.html?type=public