If you are reporting a crasher or hacker on a server that you were driving on, you need to go to that server's website directly - if they have one. Posting in this forum won't achieve anything as you can't guarantee the server admins will see it at all. The server's website can usually be found in the introduction text when you join the server, or you can sometimes find it by typing !info in the chat.
If you are reporting a post on this forum, you want to click on the little blue triangle with the exclamation mark in the top right corner of the offending post, then write a short explanation to the moderator explaining why the post is offensive.
edit: I should add that, in the case of driving incidents, it's always a good idea to have a saved replay of the incident to attach as proof when posting your complaint.