Some argue otherwise, but I think lights are essential. They're pretty cheap these days and with rechargeable batteries they don't cost a lot to run.

If you want to clip in, you need bike shoes. Otherwise, wear what works for you. I need a very stiff sole because I can't seem to keep my feet flat otherwise, but many riders are fine in regular athletic shoes.

When I was a kid, I rode many miles in jeans. Now I wear bike shorts as I find them more comfortable, but you certainly don't need them.

The other thing I don't ride without is a spare tube, tire tools, and CO2 canisters to fix a flat on the road.