There's two ways:

You can register as a reseller with any number of PC vendors (Dell, HP, Lenovo, and others) and start selling computers to customers online and/or from a retail location. Make sure you register with your local Department of State so you can not pay sales tax when you buy the PC's from the vendors.

Selling computers is not hard but it's a tough business as margins are low. It's best to also sell services along with the computers. Services such as network installation, email setup, and other things. You can even sell 3rd party services that others maintain but you simply sell or provide the lead for, such as a hosted email security service.

Ramon Ray,
Editor & Technology Evangelist,
Smallbiztechnology.com