if you mean to delete a topic (thread) then i'm not sure you can.
if you are in a question/answer type thread sometimes there is a delete answer button which you can use to delete the individual post. honestly though i only normally see it when i accidentally doublepost due to server issues.
any of the moderators can delete your post/thread so if you arent sure how just push the triangle with exclaimation point symbol on the bottom right of your post
/!\
Hi can you please delete my thread about the USB!!!! please!!!!!
and then just tell the moderators to delete your thread. or just pm a mod directly