i recently switched from windows, but i dont know how to defrag my hdd, how to di it
You don't need to run any de-fragmentation tools if you've just installed Zorin OS, as the drive has just been formatted. In fact, you're probably never need to do this as the EXT4 filesystem (used by default in Zorin OS) handles defragmentation much more efficiently than Windows' NTFS.
I wouldn't recommend you do this without explicit reason, however, otherwise you're just generating wear & tear unnecessarily.
For reference:
Modern Linux filesystem keep fragmentation at a minimum by keeping all blocks in a file close together, even if they can't be stored in consecutive sectors. Some filesystems, like ext3, effectively allocate the free block that is nearest to other blocks in a file. Therefore it is not necessary to worry about fragmentation in a Linux system.