1) Open Terminal, and cd into your app's db directory, where your sqlite database files are stored.
cd /path/to/rails_app/db
2) Run this command (obviously, you can change the parameters accordingly - the first is the name of your existing database file, and the second is the path to the dump file you want to create):
sqlite3 development.sqlite3 .dump > /Users/joshuapaling/Desktop/database.sql
Of course, this command isn't specific to rails. Just cd into the directory containing your database, or specify the full path to it as the first parameter of your terminal command, and you're good to go.