It would vary widely depending on the business you were trying to start. Basic tools would be knowledge of the product or service you were starting in, computer & computer skills, bookkeeping skills, website administration, organizational skills, and last but not least, the patience and ability to stick with it as you may not make money for 6mos - year.
Physical tools would probably be...
1. Computer, printer, phone, possibly cell phone.
2. Headset with microphone if going into telesales or customer service.
3. Sometimes you need a dedicated phone line.
4. Tools of your trade (in my case pianos, piano tuning/repair tools & supplies).
5. Business Cards
6. Office space separate from living space.
I'm sure there are more, but I have to go teach now!