The TCP 3-way handshake establishes a reliable connection. Client picks a random initial sequence number (ISN_c) and sends SYN seq=X. Server replies with SYN-ACK seq=Y ack=X+1, both acknowledging the client's SYN and announcing its own ISN_s=Y. Client closes with ACK seq=X+1 ack=Y+1. Connection is now ESTABLISHED.
After this, both sides can send data with proper sequencing. Note the +1 rule: SYN consumes one sequence number even though it carries no payload.