php mail problems

I've got a basic php mail script on my site that I'm having a few problems with.

The script sends me the content of a submitted contact form using the mail() function and it works very well, but a while ago it started sending me emails with all blank fields, which didn't worry me to much.

Recently though I've been getting emails from it that contain an email address in every field. The addresses are random digits @ mydomain.com [EG. kjhdkjh@mydomain.com] even in the IP and browser information fields.

Now I presume someone is trying to use the script to send spam, and I guess it's reasonably simple to stop but I'm reaching the limits of my php knowledge here so I'm hoping that one of you guys could point me in the right direction.

My URL is http://sounds-alarming.co.uk