The entire publishing industry is a big territory, and a lot of what I've learned (focusing on poetry and SF/fantasy/horror) may not transfer to other genres or sectors of the industry. But there are some general ideas to keep in mind.
Everything here should be taken with a grain of salt. Or a bucket of salt.
1. There are a lot of scammers out there. If you are submitting to editors/agents, a good rule of thumb is that MONEY SHOULD FLOW TO THE WRITER. That means, reputable places do not charge reading fees, or fees to publish your book or get it on store or library shelves. If someone asks for an upfront fee, or tells you that they'll publish you IF you pay their associated editing group to spruce up your manuscript, they're not legitimate.
2. If you're self-publishing, or looking for an editor and agent, and want someone to proof your manuscript or offer ideas for improvement, there are reputable people who can help with that for a fee, or with ebook design or cover art. (If I ever decide to self-publish, I will definitely get outside help with stuff like that. A good book designer or copy editor can make a huge difference in how professional the end result looks.) Approach it as you would hiring someone to put a new roof on your house, or fix your bathroom plumbing: get recommendations from people they've worked with, check out finished products, etc. Not everyone in the industry is equally skilled or experienced.
3. For that matter, do your research for editors and agents, too. Spend some time in the bookstore, and check the acknowledgments page of books you like in your genre. Many authors will mention their editors and agents, because they know good ones are worth their weight in gold. Make a list of agents/agencies who seem to have a liking for your style of story. Check out their websites and see if they're taking new clients.
4. Is an agent worth it? I have some friends who have gone without because they stick with smaller, more specialized publishers, and that works for them. My closest writer friend has hit the NY Times and USA Today bestseller lists, is going on a book tour paid for by her publisher, and has work translated and published in a dozen or more countries--and she swears by her agent. It's a complex issue, but I think having a GOOD agent can open a lot of doors. Having a sucky agent can be career-destroying. I've watched that happen to friends, too.
5. Another writer friend taught me that, when making book submissions, you should be careful to understand what that particular editor or agent is looking for, and you should not spam EVERY agent at an agency or every editor at a publisher. Many of them take the stance that, if you've submitted to one person at the establishment, you've submitted to all of them, so choose your first contact wisely.
6. ALWAYS READ THE GUIDELINES. All the guidelines. And follow them. It's pretty easy to find sites that will give you standard manuscript format (William Shunn's is the one I've most often seen mentioned), but many places will have their own tweaks or requirements. If you have an absolutely killer story, it won't necessarily be the kiss of death to make obvious mistakes . . . but lots of people write killer stories, so why give them any advantage? And there are always editors and agents who will reject you based on nit-picky issues because it's a sign that you might be difficult to work with.
7. Revise and proofread. Do it one more time than you think is necessary. Read your work out loud--it will help you catch errors that your brain glosses over.
8. If you have a genre in mind, think about joining the professional organization as an associate member, or a full member when you have the requisite sales to qualify. In my field, that's the SFWA for science fiction and fantasy, HWA for horror writers, SFPA for speculative poetry. There's also the RWA for romance writers, the SCBWI for the writers of children's books, or the MWA for writers of mysteries and thrillers. They have benefits even for beginners--RWA sponsors a lot of conventions with classes and learning opportunities, for example. Regardless of whether you write SF, you should check out SFWA's Writer Beware site--I think Victoria Strauss is still in charge of it, and it's a great clearinghouse that warns of many scammers, sketchy dealers, and collapsing presses.
9. Writers' groups, conventions, and workshops can be a great opportunity to network and learn, or a massive time- and money-sink. It's important to do your research, but also to consider what YOU need. Don't leap into something just because someone tells you that's the best way to become a success, even if they seem pretty successful to you. What works for them might be a disaster for you.
10. For that matter, be wary of anyone who tells you they have the One True Way to be a successful writer, whether that's writing every day, publishing their own work, or standing on their head while typing with only their big toe. Every writing career is unique, in both its successes and its hurdles. You're going to have to find your own path, to a certain extent.
11. Understand that this is a difficult field in a lot of ways. You have to have enough confidence to sustain you through the struggles, and enough humility to learn and grow from those struggles. Prepare to learn patience in a way you could not have imagined. And try to have a metric for success that does not revolve around number of book sales or autographs given. Otherwise, no matter how good you are, you will have too many days that leave you feeling like an impostor or a failure.
12. I tell people there are really only two rules:
1) Do what works for you.
2) Try not to be boring on the final draft.