On GoodPHPTutorials.com, there's this helpful tutorial that anyone working with any sort of website that has a database backend should look into - a brief look at SQL injections with PHP and MySQL.
SQL injections are a major security risk in many PHP applications. Injections are caused when a web developer allows the end-user to manipulate a variable that is being inserted into a database query string, generally through the $_GET, $_POST or $_SESSION superglobals. When a value isn't verified, major problems can occur. Since MySQL is the most commonly used database platform for PHP applications, it seemed appropriate to write an article specifically related to that.
They cover different topics that you'll need to protect yourself against a basic level of SQL injections - the key is the validation of your data. They have code examples to help you along and explain each step, including outputting the data to the page (where things like cross-site scripting issues can occur).