Go into the upload contacts in the Database Menu (shown below)
Upon clicking you should get a pop up window that looks like this:
-----Excel spreadsheet----- [RECCOMMENDED FORM OF UPLOAD]
If you are uploading an excel spreadsheet, make sure you have it saved in CSV format.
You can view how to do this in the 'How do I save my excel database as CSV?' article in the database section.
If your file is already in CSV format, then the columns are already separated by commas. Once you have selected the file you can now click the Upload button.
If you are uploading a txt file, you will need to specify how the columns in your file are seperated.
The file will not be instantly uploaded and you may have a wait a minute or so for it to complete but you will be notified the moment it is uploaded.