IU is a huge school with over 40,000 students (Undergraduate and Graduate) and while that may seem intimidating at first, it just means that there are so many things to get involved with and so many people to meet!
At the beginning of the school year, there was a big club fair held in the parking lot of the Union. A lot of different organizations and clubs were there to inform students about what they had to offer. I went and I'm so glad I did. I learned about clubs that I had no idea even existed, got some free stuff, and signed up to join a few things. However signing up does not commit you to anything. It's perfectly okay to choose not to get involved in those clubs. I went to a few call out meetings that first week to see if I was interested in them. Call out meetings are great, sometimes they have free food and it's a great way to meet members of the club or organization and find out what it's all about.
I joined the Student Alumni Association (SAA) and the Student Athletic Board (SAB). Before going to the club fair, I had never even heard of those two things, but now I know so much about them. SAA helps plan lots of different events around campus (they do a lot with Homecoming) and it connects students and alumni. This link has more info on it if you are curious! http://alumni.indiana.edu/affiliates/student/index.htm
SAB is a great way to get involved with sports and it's a lot of fun. They help out at all the sporting events doing everything from throwing T-shirts to the crowd to meeting and greeting VIPs. You can choose which sports to volunteer for and what are you want to focus on. I helped out at a few football games and I got a credential (I felt super cool) and it was a lot of fun!
Besides just clubs at IU, there are tons of organizations (shout out to IU Hillel!) that are really awesome and have great activities and events. And the club fair is not the only way to find out about these clubs and organizations. There are many things that they use to spread the word from chalking, tabling, putting up flyers everywhere, Facebook event pages, mass emails, etc.. I definitely suggest looking into everything that IU has to offer. Below, I have posted a link to a page on the IU website where you can find out about all sorts of clubs and events that are happening on campus. And if for some reason, you ABSOLUTELY can't find something that interests you, you could always start your own club!