Some tips for preparing your import file:
1. DESC is required field, FULLDESCR is not.
You can move text from FULLDESCR to DESC
2. PRICE field should have only numbers without "$" sign.
3. THUMBNAIL filed should have exact image name (images have to be already on the server)
Go to "General settings" -> check "Check this to close your shop temporarily"
Go to "DB backup/restore" -> Check "Write SQL dump to file" -> click "Generate SQL file"
This will save your full database to the server. If something went wrong with the import you just click "DB backup/restore" -> "Restore" to get everything back.
Click "Import products"
Check "Get columns names from the first row of CSV file"
Select "Comma" in "CSV delimiter"
Beside "CSV file for upload" click [Browse] and select file to import from your computer.
Check everything above again and..
Click [Import products]
If import was successful, go to the store home and check if new products and thumbnail are there. If something is wrong - restore your database as soon as possible and do not forget to open your store in "General settings".