How do I make my avatar dance in Second like?

The best way is to get what's called an AO or "Animation Overrider"

You might need to get some separate animations. There's a place called SineWave that has some great dances.

Use the search feature to find these things.

If you can't find or get an AO you can run an animation straight from your inventory manually by just double-clicking it... You will be presented two options, "Play Locally" and "Play In-World"

Play locally will make it run for your computer only so you can see what it does before you embarrass yourself

Play In-World makes it run so everyone can see