I've never lost a loved one in a terrorist attack or anything similar, but in July of 2000, my house burned down. I had gotten up early one morning to go shopping, but my three roommates all died in their beds. The fire was caused by faulty wiring, and apparently had been burning all night within the walls, unbeknownst to us. We had just moved into the house, and the landlord did not provide smoke detectors. If I hadn't left the house early, I would surely have died too. It was the worst thing that ever happened to me, and this is some of what got me through...

Talk about it when you need to. It's going to to be in your thoughts for a long time, as you turn it over and over examining all of the varied "what if *this* had happened" or "how can I keep myself and everyone else safe in the future" type thoughts. If you don't have a sympathetic ear, write it out, instead. It really does help.

Conversely, if you *don't* feel like thinking or talking about it any more, don't be afraid to say so. Too many sympathetic and well-meaning people at once can be very, very overwhelming and answering the same questions over and over can be trying. Feel free to simply say "I don't want to talk about that any more right now."

If you or a loved one were directly affected by the bombing, accept help in the spirit with which it is given. The goodness and caring of people in general will touch and amaze you.
Help someone else who is suffering. It takes your mind off your own situation, and gives you a measure of control and hope to be doing something positive. Pick out one of the people lost in the attack and start a college fund for their children. Create a memorial for someone you know that died. Offer to host the domain names of a WTC company who lost their webserver and create a page for listing those company members who are safe. Just do something, no matter how trivial it seems at first.
Plan and organize your immediate future. This is something else that will restore some of your feelings of control over your life.
People are very strong and resilient, and no one can mourn 24 hours a day forever. If you catch yourself laughing or smiling, don't feel guilty, as if you are too easily forgetting those that were lost.
And just remember that everyone deals with things in different ways, and nothing you do is wrong. You know better than anyone else what you need to do for yourself.