Be excited about sets and logic, and generally what you are talking about.
When I was a freshman I had a TA in calculus 2 that was totally awesome. Not because he was particularly good, and I was particularly uninterested in the topic. But to hear him talk about the theorems was inspiring.
I took from that a lot, and when I was TA'ing intro to logic and set theory, I too tried to be excited about whatever it was that I could be excited about (that is, not the extremely dull theorems, but most of the other things). I would pick exercises which I found exciting, then it was just so much easier to get excited.
The important things are:
Keep the class involved. Ask them a questions and wait for them to answer. When my students don't answer I just stare at them and tell them we're not going to continue until they do. I sometimes use an application on my smartphone to make sounds of crickets when they are too quiet, they usually laugh and then they answer.
Use examples that you think are awesome. Examples which you think will surprise them. Things they are expecting to be true will bore them, and they won't be sure what there is to be excited about. But if you catch them unprepared then they will have a better chance of enjoying the class.
Spice things up with history. Who proved that, peculiar notations from the history of the topic. Don't overdo it, but from time to time it's nice to add some background, especially if people's names are already mentioned.
All in all, teaching is much like story telling. You tell a story, and they listen and learn from it. If you think that the story is dull and uninteresting, then your crowd will think so as well.