I started 6 months ago on a cheap Magna Excitor ($115) and upgraded to a Specialized Sirrus ($450) in 3 months. Don't make my mistake, get a good bike. Needn't be expensive, most here would look at my Sirrus as cheap. If you know your size and buy used, you can get sonething like a Sirrus as low as $200 - $300.

Only essentials you need to get at the beginning is a patch kit, tire levers and frame pump. Even those may be optional .. e.g., I still don't have a frame pump. My commute is just 3 miles each way, so the maximum I would need to walk is 1.5 miles. Include lights if you ride in the dark. You should use them even during the day, but my commute is simple so I don't.

The rest .. rack, bag etc can come later.