There are a million books on parenting out there and reading them can be really worthwhile, not just for raising kids but for thinking about emotional and social landscapes. More than that, though, it's never too early to start really thinking about parenting philosophies and approaches. You won't be able to prepare for your kid, but you can start to form opinions on which approaches sound good to you and which don't. By the way, that absolutely includes approaches that sound effective but just not like something you can implement, for whatever reason. One thing I've learned is that parenting tactics aren't going to be effective if they're not genuine, and that's one reason that so many parenting philosophies exist and claim to be successful. In reading about this sort of thing, you'll learn a couple things.
First, you'll get familiar with some of the most common topics you'll run across--particularly cry-it-out and other sleep training techniques, discipline, communication, handling tantrums and other big emotions, &c.
Second, you'll find yourself really starting to think about your own experiences and about a lot of your relationships--how you talk to them, how they talk to you, and how you handle conflict, among other things. (You also get to start noticing how many adults have tantrums and what kind of crap excuses we all use to pretend we don't!)
Finally, you'll hopefully start to notice some themes running through all of them and that will help you identify some rare "parenting truths" while putting together a toolbox incorporating suggestions from lots of different areas. Once you have a kid, you'll likely be surprised at which sections you find yourself digging into, and you'll very possibly change your mind completely on the stuff you settle on beforehand, but that's constantly true as your kid grows, anyway.
Two other pieces of advice: 1. Read up on medical issues surrounding pregnancy, childbirth, and childrearing. Not, like, go deep into all the scary stuff, but the less you're surprised by the topics and decisions you'll come across, the more confident you'll feel making those decisions. 2. Try to keep your eye on the prize--remember that you're parenting a child. The reason we talk about owning a dog versus raising a child is because the dog will continue to be a dog and you can train it but it's always going to depend on you, basically unchanging, for its whole life. With a child, the important thing is the adult you're raising the child to be, not the measure of the child at the moment against an ideal of adult behavior. So, for instance, sure, be considerate of other people (regarding crying in restaurants and tantrums and running wild and all that), but resist disciplining your child based on what the other people around you at the time would like your child to do or not do. Sometimes you gotta get groceries with a miserable kid. Make sure your interactions are based on what the kid needs to become the person you want them to be, not what someone else wants because they don't feel like hearing the crying or they don't approve of the decisions you're making. I try, as one of my highest goals, to make sure my kids know they come first for me, and if they're feeling vulnerable or scared or otherwise awful, I will be late or leave the restaurant before our food gets there if that's what it takes to give them the assurance they need to make their way in the world. I will be firm on rules but I won't tell them what they're doing is wrong without a reason, and if I can't think of a reason, I'll reconsider whether what they're doing is fine, because I want them to grow into capable adults who are not easily swayed and who can think critically. It's never too early to practice contextualizing how you plan to parent in terms of the effect it will have on their ability to understand and predict how the world works, and on their ability to make and trust their own decisions, and on their sense of how to take care of themselves and interact with people with confidence. Corollary to 2: It feels like a lot of people buy into "advice" like "Sleep train your baby or they'll never stop sleeping in your bed/Take away the pacifier before 1 or they won't be able to soothe themselves without it," but are perfectly willing to tell their kids, "Because I said so." For me, parenting is mostly about emotional guidance so my kids aren't buffeted by every wind, and the rest is logistics. So, no, they're not going to sleep in my bed in college or go to bed in their first apartment with a milk bottle. I don't really care when those things happen and generally try to take the path of least resistance--is the weaning or potty training making us all miserable? It can wait. Can I not bear to search for one single more bottle nipple? Then out they go. But kids can grow into adults that throw tantrums, refuse to share, think they don't deserve to get what they need or won't get it unless they just take it, and don't know how to determine whether what someone's telling them to do is right or wrong. Those are the crucial things.