Hi Saluki
In addition to what SABTeacher said, I also wanted to mention that you're really not supposed to speak until you're spoken to, and it's a very somber event. Generally, conversation is kept to a minimum and you may notice that visitors are sitting in silence, which can be ackward for those not familiar with shivas, but is perfectly acceptable. Again, this may vary depending on the family's background. You may notice people are sitting either low to the ground or on the ground.
HTH~