A website I am building currently stores and displays information in a SQL database and I am concerned about how safe my data. How easy would it be for someone to break in and does any one know of any guides that would let me have a go at it myself.
Amazon.com: SQL Injection Attacks and Defense (9781597494243): Justin Clarke: Books
Amazon.com: The Web Application Hacker's Handbook: Discovering and Exploiting Security Flaws (9780470170779): Dafydd Stuttard, Marcus Pinto: Books
If you want something free look at the OWASP Testing Guide (or just the OWASP site in general):
Category:OWASP Testing Project - OWASP
There are also various training courses on the subject, including the PWB course from Offensive Security and the GWAPT and GPEN courses from SANs.
I have only given very high level answers here because this is not the type of question that can be usefully answered in a forum post. If you dont already have skills in this area your best bet is to hire a professional to assess the security of your website for you.