Definitely seek out playgroups if you feel yourself getting isolated or your public library should have storytime once a week for your child's age range. Staying at home was a very big adjustment for me and when I look back I wish I had seeked out playgroups, storytime, gymboree type of stuff. It probably would have made the transition a lot easier.
You're going to love being a SAHM. It's the most rewarding career there is, IMO. Yes, you will have rough days, but being able to be with your child every day makes it so worth it. I thought it would be rough, considering I've always enjoyed working, but for me, it's been wonderful.
I agree with PP, though. I keep my sanity by not staying in the house all the time. Get out with your LO and enjoy all that your area has to offer.
Good luck, and enjoy the adventure!