A good technique for newcomers is to decide on one or two (not more!) WGs whose topics are interesting or relevant, and join their mailing lists.
You can find the list of currently active WGs on the IETF Datatracker. Clicking on a group acronym (e.g., "tcpm") gets you to that group's list of documents. You will want to click on the "About" tab on that page to see the group description. From there, in the "mailing list" section in the table at the top, there is a link "to subscribe" to that groups email list. Follow the instructions there to sign up to the mailing list and you have effectively joined the working group.
A working group's About tab might also have pointers to other sources of WG information, such as a wiki page or a Github repository.
If a WG is in an active phase of discussion, you may well receive tens of message a day from each list. You need to be using a mail program with a good method of automatically sorting incoming mail into multiple inboxes.
Once you have that working, simply read the mail threads daily, and read any draft documents that they refer to. (See the next section for hints on how to find particular drafts.) It's advisable to wait several weeks before starting to contribute to the mailing list. But when you feel comfortable that you have something new to say, go for it!
The IETF is normally very welcoming to newcomers, and tolerance is the rule. The technical level is quite high, so if you write something that turns out to be wrong, you may get some quite frank replies. Or sometimes you will get a reply from someone whose first language is not English, and they can be rude without intending it. (If someone is seriously offensive, the WG Chairs are supposed to deal with it.) Don't be discouraged; everybody started as a newcomer.
If you are just getting started in the IETF, you may be interested in the IETF Guide Program. This program matches experienced IETF participants with newcomers in order to aid their integration into the IETF community through advice, help, and collected wisdom. The guidance provided by the mentors should speed up the time it takes for newcomers to become active, contributing members of the IETF.