If we are talking essentials: you only need clothes that are comfortable to ride and exercise in, a drinks bottle (your camelbak or similar will do just as well to start) and tools to repair a puncture (repair kit and pump (+ a spare inner tube will make any repairs easier)) or mechanical out on the road. If you're intending to leave the bike anywhere, you'll need a lock. You need something on the bike that can hold all this stuff and anything else you take (keys, mobile phone, food if you need it) that can't be carried in your clothes.
You only need lights if you're going out in the dark. A track pump is one for down the line IMHO - at this stage as long as you have some form of functioning pump you'll be fine.
There are lots of things which you will later purchase that you wondered how you did without but right now I can't think of anything else that is "essential".
Personally I would add a bike comp (cos I like to know how far I've been) and cycling mits (for comfort and because if, god forbid, you fall off it's normally the hands that bear the brunt).
HTH.