The documentation for join has a nice write up on the command that should get you started. In general for these types of questions I start with the Splunk Documentation. The docs teams does a good job.
One cautionary note: I would carefully consider if you need a join or not: there are a lot of other commands that can solve the problem where one would normally think they needed a join. Also, many of us with a SQL background seem to jump to join more quickly than they should because the terminology is familiar. There's nothing wrong with join; it's just that it's about the slowest and most "expensive" way to do what it does. So, if there's no way around using join then by all means USE it, but it can be worth it to ask in another question "I have this search with this join and it runs slow. Can someone help me rewrite it more efficiently?" (Obviously, a small set of sample data and a good description of what you are trying to accomplish is important there, too).
Anyway, good luck!
