Once you entered in the page you have to select you media target.
Then, you have to upload the file you want to convert in the chosen target format and click the "Convert file" button.
Wait until the progress bar reachs 100 %, this could take time depending on the file size.
Once the file is already converted, a download dialog box will appear with your new file, you know what to do with it haha.
After downloading you file, the site will show you alternatives to be selected, you can select "Convert another file to ..." if you want to.