Firstly, congratulations! Secondly... don't panic and believe the horror stories (don't people just love sharing those?).
Toddlers aren't exactly known for their ability to share, to reliably be kind or to not throw things however will a little bit of guidance from you it can make the whole, overwhelming change in their life as they know it so much easier.
1) Talk to them about the baby- ALOT
Get them excited about the baby, get them talking to other people about him/her and allow them to think about them as a real person.
2) Make sure that you are giving them time away from you. Once the baby arrives it will be so very hard to continue giving your firstborn the amount of attention that they are used to so start now- allow them to spend a little more time with your partner, their grandparents etc so they are still receiving good attention but it isn't solely received from you.
3) Teach them the art of patience. They are going to need to learn this, try making them wait just a little longer than usual for some of the things they ask for e.g. waiting until you have finished your coffee before getting them a snack.
4) . Show them photos and especially videos of them as a baby. Even better if it is from the same house, that way they'll be able to understand a little better and, depending on their age and understanding, learn that infancy is just a temporary stage.
5) Once baby has arrived try to take some time out (during baby naps/ if your partner can take baby for an hour) to spend some one on one time with your eldest.
6) . The big one- sharing. Start now, make them share their toys, praise them for being kind doing this and talk to them about why it is nice to share with others.