If you'd like to learn more about TCP, you can look at the RFC that's available from the IETF at https://tools.ietf.org/html/rfc793:
You're looking for RFC: 793 for TCP, which is the original specification for TCP.
In the preceding screenshot, you can see different sections within IETF, which provide a little bit of interactivity. You can click on the different RFCs that have updated the TCP specifications; if you scroll down, it also provides you a nice little table of contents. The RFC shows a little diagram of what the TCP header looks like:
We have Source Port; Destination Port; Sequence Number; Acknowledgment Number; Data Offset; some Reserved bits; Window size; header Checksum; an Urgent Pointer; and Options, which is an expandable section. We have some Padding and then the actual data.