On left hand menu, search "Data Import Wizard" or go to Administer -> Data Management -> Data Import Wizard
Click the "Launch Wizard!" Button
Under "What kind of data are you importing?", Click "Standard Objects"
Click "Accounts and Contacts"
Click "Add new records"
Add the CSV
Map any unmapped fields
If everything looks like the image below, click Start Import
When done (Progress 100%), you will see records processed.