Out of curiousity, if you only have a Mac, and know absolutely nothing about the MOBA genre, where would you start?
Sadly, you are out of luck for a bit. Valve has confirmed a Mac client for Dota 2, it is just a matter of when it will be released. Riot killed the League of Legends Mac client, a knock off one has popped up over at boompje.net, you can try that but it has a lower frame rate and can be unstable after each patch (which happens at least once every 2 weeks). The biggest thing you could do is install Windows on your Mac via Bootcamp, that is what I do and play all my games on the Windows partition.
As far as stepping into the ARTS (or moba) genre, League of Legends is a much much simpler version of Dota. You would be able to pick that up quicker and then make the transition to Dota 2 if you choose to. However, if you are able to get access to a Dota 2 key I would suggest jumping in deep. You will need to accept that you suck and need to read / study a ton of guides, watch pro matches...etc then you will be able to grasp what is going on. In the past few months of Dota 2 I have racked up over 500+ hours of play time and I am not even close to considering myself a top tier player. Watch some games over at http://www.joindota.com and see if you find the game enjoyable enough to dive into, there really isn't anything else out there that is as competitive as a Dota match.
Heros of Newerth does have a Mac client, however that game will really turn you off of the genre. It looks ugly, player base sucks, and you really wont enjoy it. If you are really desperate, you can give it a try but I higher recommend against it. Hope this helps, let me know if you have any more questions.