I'd recommend you man chmod to get some more info, but here's some basics for ya. Your command should look something like this...
chmod 755 /home/username/desktop/trash
The numbers(755) after chmod assign what Owner, Group, Users can do. The first digit is for Owner of the file. The second digit is for the group, and third is for everyone else. The number used determines what they can do. For example, 7 means they have full access. They could delete it, change it, etc. Once again, I'd man chmod to get more info on which number you want to use.
I forgot to mention. You may also need to do something like...
chown username /home/username/desktop/trash
The user should be the owner of the trash folder, and most things in their /home/username for that matter. But I suppose it's possible that root is the owner of that folder.