Malware--help, please!

My computer seems to have gotten one of those things that sends spam which appears to have come from my email address. Is there a way for me to find and get rid of it (I have moderate facility with ordinary issues; don't really know how to do any of that console stuff without very specific directions)? Or can you please suggest inexpensive software that will do it? I've never had any problem of this type before in many years of Mac use. Help, please!