congratulations on having the opportunity to stay at home with your son. My Daughter is 20 months and there is no way I would put her into Daycare. I do taker DD to a couple of playgroups a week (when I can) I also run a playgroup at padstow in NSW. I find that each group is very different, and often groups can be very cliquey, but try and find one if you can, as will help both of you. In the long run your son will be watching you to see how you interact with new people making friends, that sort of thing. I myself don't have any great friends from playgroups (that I met there), but I do get to socialise with them once a week. It also helps teach my Daughter sharing that kind of thing.
Also the things with playgroups is there are very different set ups. Some are more a mums social group, with no structure, so the kids just play with each other. Some are structured with singing/story time, Morning tea, craft, free play etc. SOme playgroups will roster you on to do things, like help with morning tea, set up craft etc. others will have volunteers who do everything and others will be a mix where everyone just helps out.
Hope this helps somewhat for you both.
Remember you are the best teacher for your child.