You can't just say "How can I make my site secure?" and then we'll tell you how to secure it - it's a bit more complicated than that. You haven't even given us a URL
Regarding SQL injections, it's a huge area, but if you use something like
PDO and prepared statements, you should be fairly secure.
If your website involves money, you may want to consider paying for a security audit. From what I've heard, it's fairly expensive - I've heard $100 an hour at 1000 lines of code an hour, but it will obviously take less time if it is well documented and well laid out. You may be able to hire a nub like me to do it for you. It'll be cheaper, and not as good quality, but they would probably get the job done.
~Callum