After enabling TRIM should I delete free space? I have installed a third party SSD in my Mac. I have used it quite a while before I enabled TRIM. 

Does that mean that the SSD cannot reuse any files I have deleted until now? Is it necessary to somehow delete the free space of the disk to tell the disk via TRIM that this is really free space now?
If this is not your system disk, you should kick off a disk repair which will trim unused blocks. Otherwise, boot in single user mode (hold ⌘+S during startup) and issue the command:
fsck -fy. Be aware this will not work from rescue systems, as they do not have trimforce enabled, while booting to the single user mode acknowledges trimforce.