There are many free PC/Mac remote control services such as GoToMyPC. There’s also VNC, which is open source version of those. You will need to install server/client software on both ends of the computers, just like GoToMyPC. Using Chicken of VNC, you can now remotely control your Mac and here’s a great site detailing how to do that.
One more note, you can use free services like no-ip.com, which allows you to maintain a static domain name for your dynamic IP address. This will work well if you are on regular phone DSL as they tend to change IPs often.