The following video covers formatting basics in Google Sheets, using the Google Sheets toolbar. Bold, text color, fill color, and other options are demonstrated. The instructions apply to the new version of Google Sheets on a PC, Mac, or Chromebook:
Note that formatting options are far more limited on the Google Sheets app for iPhones, iPads, and Android devices.
For more tips and tricks about Sheets, check out the latest edition of my guide, Google Drive & Docs In 30 Minutes.