The IETF has no formal membership, no membership fee, and nothing to sign. By participating, you do automatically accept the IETF's rules, including the rules about intellectual property (patents, copyrights and trademarks). If you work for a company and the IETF will be part of your job, you must obviously clear this with your manager. However, the IETF will always view you as an individual, and never as a company representative.
The resources below provide a starting point for learning more about how to participate in the IETF. There are also resources available if you would like to learn more about RFCs, the documents produced by the IETF standards process. More information about upcoming IETF meetings and past meetings is also available. "Running code" is an informal mantra of the IETF, and IETF Hackathons are a great way to get familiar with and help develop IETF standards.