Some employees at the office have been using an iMac, and now I am the only one who will use the station. I want to clean it up from the former user's data. How can I do it?
On Mac OS X, it is customary to store all personal data in one's own home directory, so deleting that should get rid of most of the junk. Pretty much all applications with very few exceptions store users' personal settings in a user's Library within the home directory.
To remove the other users' home directories, open System Preferences, go to Accounts (or Users & Groups) preference pane and delete the other user accounts. Confirm when asked whether to delete their data (unless you want to store it somewhere).
You might want to take a closer look at non-Apple stuff in
Some things I wrote here also apply across user accounts.
If the other users had administrative permissions, they could have changed a lot of things deep in the system — e.g. installed a lot of Linux/Unix software, possibly compiled by themselves. There is no sane way to get rid of it all except wiping the disk and reinstalling the OS.
If you want to erase the old OS X installation completely, erase and reinstall OS X from the recovery partition (in 10.7 and later) or from an OS X DVD (in 10.6 and earlier). See these Apple KB articles:
If you are selling a Mac, you should erase and reinstall OS X instead of deleting parts of the current installation manually.