I give this advice based on the assumption that you didn't put system files and folders into the trash bin
Open up a terminal window and type "ls -la"
This should show you all the files and folders in your home directory (including hidden files)
You should see one called .Trash - this is your trash bin.
Type "cd .Trash"
if you run an "ls -la" this will list everything currently in your trash. Confirm that everything in there is ok to be removed.
If there is something you want to keep, you will need to move it to your external storage, this can be done by typing "mv <name of file> /location/of/external/drive". You can get this by typing "mount" in the command line.
Finally, with that all done, type the command "rm -r *" - this will delete everything in your trash (so make sure you are in the trash directory)
That will clear out your trash and hopefully give you a bit more room on your hard drive to work with