Getting started is easy. You can take a look at the Apps Script Welcome Page, and then work through some of the Tutorials. Before long you'll be writing scripts to automate all kinds of repetitive tasks like keeping track of group expenses and sending out automated reminders triggered by values in a spreadsheet.
In this example, we're using a Google Docs spreadsheet to keep track of our soccer team's weekly dues. Each player owes $5 per game towards shared expenses. Unfortunately the players can be an absentminded bunch that need frequent payment reminders. In the spreadsheet, every amount entered updates a player's column to indicate which weeks are PAID (in green), and how much is DUE (in red). We've also used Apps Script to add a custom menu:
'Refresh' recalculates and colors cells to indicate which weeks are PAID and which are DUE
'Send Reminders' emails a polite message reminding players in debt to pay up
Go ahead and take a look at the sample code in this template. After choosing Use This Template, select Scripts... from the Spreadsheet's Tools menu. It's a very basic script, but it gets the job done!