Sourcerer may make things quick, but spend some time looking into plugins, modules, templates and components. That's the right way of extending the Joomla framework. There are plenty of tutorials on creating these on Joomla's website.
For a single form, a module is probably what you'd need.
Sourcerer adds lots of display code to your database, which really isn't very good from the perspective of keeping things tidy. Trying to tidy up or re-skin a site built using sourcerer is a nightmare!