Each side of a TCP session starts out with a (relative) sequence number of zero. Likewise, the acknowledgement number is also zero, as there is not yet a complementary side of the conversation to acknowledge.
(Note: The version of Wireshark used for this demonstration, 1.2.7, shows the acknowledgement number as an apparently random number. This believed to be a software bug; the initial acknowledgement number of a session should always be zero, as you can see from inspecting the hex dump of the packet.)