There are as many mistakes as there are different tanks, no setup is the same.
1) be sure to cycle your tank.
2) before adding any fish or cleanup crew do a 50% PWC
(partial water change), this will be the only time you can do this without worrying about your animals.
3) add life to your tank slowly to let your bacteria catch up to the new bio-load.
4) Be sure to buy a liquid test kit and a refractometer to test your water parameters.
5) research your tank inhabitants before buying them.
6) Remember this: the only thing that happens fast in a saltwater tank are "MISTAKES"
7) read, read and read some more.