If you're using a modern browser, you can drag files to the upload area, or click to pull up a file browser. Otherwise, you'll see a traditional file upload.
Columns are identified by the headers in your file. Headers should be on the first row of your excel spreadsheet, or on the first line if you are using a text file.
Files are limited to 2 MB. The valid files types are Excel (.xls or .xlsx), text (.txt), and csv (.csv).