Joomla Website Testing and Minor Functionality

Hi, I have a site which I built with Joomla and I need someone to test the website for any bugs, errors and if necessary add any security to the site - it seems that someone may have hacked or interfered with the site but I'm not sure. Some users are having trouble registering with the site or adding their content.

I also need some minor functionality added which I gather requires some hard coding.