I need help signing up for iTunes Match
The first thing is to check that your iTunes is new enough for the subscription service. You didn’t mention how old your OS is so if you need help figure I get that out, Apple offers presales support via phone and web chat. 

I believe iTunes 10.5 is required, so try to update your Mac OS and then iTunes as far as you can to current. 

Next, if you’re sure your iTunes is ok, it’s likely a payment or date/time error. Again, without some specific details we might struggle to help you out further.