WHAT You Need
Step 1. Pick a song. Step 2. Embarass yourself?
Even if you're not a good singer, there are ways around this. Bob Dylan songs for instance. Or songs that are intentionally done bad (Backstreet Boys, NSYNC, etc).
However, the easiest genre to do in my opinion, is rap.
Rap is the easiest Karaoke option to be "good-shitty" at. Singing, on the other hand, you have to be somewhat in-tune, find spots to breathe, use proper inflection, etc. There's a lot to think about with singing while with Rap, you can just basically talk in a structured cadence and people will be ok with it. With Rap, as long as you know (most of) the words and have a (somewhat) steady cadence... easy, done. I'd suggest "old school" songs as well like Warren G - Regulators. This is a crowd favorite from what I've experienced. Everyone knows it and it's not too difficult to perform.
Are there any songs you can think of that would make a "good" karaoke option? My personal favorite is SkeeLo - I Wish. (feel free to steal that one and use it for yourself--another crowd pleaser that poeple will sing along during the chorus with)