So you get pregnant, do a lot of research, obsess about all the fucked up things that can happen to your baby, plan the perfect registry, write a birth plan, find out that you can't control child birth much more than you'll be able to control your mini at 16 and get home with a bundle of joy. Maybe you have some friends with kids. Or maybe you don't. It really doesn't matter since the ones with kids already have their schedules set up and, as you'll soon know, there's a world of difference between a 3 month old and a 13 month old. Your friends without kids will probably be easier to meet up with anyway assuming they don't mind having distracted, wandering conversations with you while you restrain your child from stuffing sugar packets into her/his mouth.
What you'll soon discover is, you need to meet some other mamas. Early on, breastfeeding or new mom support groups are handy. No one's kid does much yet, you're all shellshocked and happy to get out of the house. These will probably not be the people you stay friends with. When babies are really small it's all about changing diapers and making sure they're eating enough. As they get older, you'll find out that the nice woman you always had friendly chit-chat with is actually a big fan of some strict child training philosophy.
Mom's groups and Mommy and Me classes come up next. By about 4 months, you'll have learned something about your local mom culture and gotten some leads. I am not a joiner (actually I'm an eternal optimist when it comes to groups and always join a new one thinking this time it'll be different) but have (surprisingly enough) had luck with a mom's group. Important point here: you need to know your demographic and make sure the group you join won't annoy the hell out of you. There are all kinds of battles in the parenting world: breast or bottle, at home or working, cloth or disposable, attachment parenting or child training, co-op or drop off nursery schools. Though most of it is ridiculous and, chances are you'll never follow any one philosophy absolutely, people get really strident about their parenting choices. It's good to remember this going in. Also helpful, trying to be mellow about other parenting styles. One of the things I've learned since my daughter was born, is that you can have all the fantasies you want about childrearing but in the end, your child has a unique personality from birth and you have no idea what's going to work for the two of you until you get there.
If Moms groups aren't your style, Mommy and Me classes or storytimes can also be great. Gymboree, library storytime, one of those stroller fitness classes - something you can live with that will help you meet mamas you like. The main thing is to find a mama or two, more if you're lucky, that you'd be friends with even if you didn't have kids that were about the same age.