Wow. LOT of opinions on here. Many of the things listed I wouldn't call anything close to essential. In my opinion, like a few others have said, you have a bike, and the desire to ride. Everything else, you'll figure out.

For example:

I was riding w/ a bag & a lot of books- didn't like the weight on my back, so I got a rack & pannier.

I was riding w/out any patch kit or repair tools, then some friends got flats & I decided I'd rather be prepared, so I got what I needed.

I always had a helmet & lights, but one rainy night I went over some railroad tracks, flew off the bike, and landed in traffic. My helmet hit the pavement, instead of my head, and the lights prevented me from getting hit by cars. Now I don't even question using them.

I didn't like riding on platform pedals, so I got straps & cages. Didn't like those either, so I got clipless, and I love them.

Just roll with it (no pun intended), you'll figure out what you need. Good luck and have fun!