My last pregnancy advice post was about what I suggest you do before you get pregnant. So it follows that I should discuss what I advise you do when you discover that you're pregnant.
1) Keep taking prenatal vitamins, or start immediately. The folic acid in prenatal vitamins helps prevent neural tube defects in the first trimester.
2) Make an appointment with your OB. (If you don't have one research a good one.) Most OB's won't see you until about 8 weeks because they honestly can't tell you much before then. Record the date of your LMP so they can determine your due date.
3) I don't think you should tell everyone that you're pregnant immediately. Obviously it's your personal choice to make and you can tell anyone you want, especially if you're close to that person. But be aware, approximately a fourth of pregnancies do not work out and you probably don't want to explain to everyone if yours is one of those. Even if you are blessed with a healthy pregnancy, a lot of people will feel really uncomfortable with your early announcement, either because they are worried for you or because they experienced a miscarriage themselves.
4) Don't immediately run out and buy a ton of stuff. Largely for the same reason as above, but also, towards the end of your pregnancy some loved one may want to shower you with gifts.
5) Related to the above, ask a friend who has a young child for help making a baby registry. (I might do a post on what baby stuff to get if I don't run out of time.)
6) Sign up for babycenter.com or thebump.com. Skip "What to Expect When You're Expecting" and the like. If you want to buy a "pregnancy book" I recommend "Your Baby's First Year" by American Academy of Pediatrics. Unlike the authors of some other books, these people are doctors. There's a solidly long pregnancy section in the front, followed by an infinitely more useful section on what to do when the baby is actually here. It's like two books in one.
7) Please take belly pictures. I know a lot of moms who didn't because they were unhappy with how they looked at the time, but you might not always feel this way in retrospect. So if you don't like how you look right now, take the pictures and put them somewhere safe without sharing them. Later in your life if you still hate them you can burn or delete them. But most likely you will like the photos later- or your child will.
8) Don't get too attached to your "birth plan." It may not happen that way at all.
9) Don't eat too much junk food, especially if it has preservatives. The baby does not "need" that, and you'll find most of the weight you gain in the first and second trimester goes to your bum and not to the baby. That's weight you'll still have on your bum after you have the baby.
10) Prioritize sleep as much as you can. (This is hard, especially if you already have another child.)
11) Be easy on yourself. You just can't do everything you used to do. (This is very hard.)
12) Morning sickness can sometimes be avoided by not letting yourself get too hungry or stressed. Other times it can't be avoided at all.
13) Tums can be really helpful for acid reflux, but beware they can cause iron deficiency. (Learned this the hard way.) Iron deficiency can be really serious because you may lose a lot of blood when you have the baby, and really low iron could result in difficulty breathing and might even result in you losing consciousness.
14) Research any medication (prescription or over-the-counter) people suggest. You should understand the benefits and risks before you take anything (including prescription prenatal vitamins that often contain "extras"). Here's a link I found explains the issue really well: http://blogs.plos.org/publichealth/2014/03/03/tylenol-safe-pregnancy-trick-question/
15) Now might be a good time to write a will, determine who will care for your child if something happens to you and your spouse, and even write an advance health care directive. I'm impressed if you're motivated though, it took us almost 3 years after James was born to do our will.
That's all I can think of, but comment if you have any questions and I'll try to offer what I can.