This.But really, cycling as with most things, is best started in the most minimal manner possible while maintaining safety so that you can save money and figure out what you really need and what other things may just be wants.
If you're going to be biking farther than 10km you'll probably want a proper bicycle for pavement, mountain bikes get old fast.
For instance: I started cycling the 35km each way to work and had a helmet and a bicycle and a backpack full of stuff I needed for work, including my cell phone. I wore a technical running t-shirt and some basketball shorts and used running shoes. I found the following after the first day:
All vehicles hate me. I immediately, 11 kilometers in and 6 kilometers from a gas station, had a catastrophic flat tire and bent my front wheel and my chain broke. I called a friend and he gave me a ride to work. A patch kit, new tube, tire levers, a wrench, and hand pump were essential. My partner doesn't carry any of this stuff and has had no problems. She's a lucky lady I guess.
After the second day, which was a great success, I discovered the following:
My hands were DESTROYED by road debris. Gloves were needed.
My eyes were SORE. I'm pretty sure I have a significant amount of the topsoil of New Brunswick inside my face still. Protective sunglasses were needed.
My feet were numb and painful. Cycling shoes would help, but I could keep biking with sore feet so I toughed that out for another few months to save for it.
My partner has no problems with the shoes but does use sunglasses and gloves as well.
And that's pretty much what I need. Things that I want:
Pannier racks and bags, cycling clothing, better gloves, a helmet intercom.