Lots and lots of research. Saltwater can be a challenge in a large tank, when people find those too easy they get a smaller tank for the challenge.
The most important things are:
1-lots of live rock, at least twenty pounds.
2-Keep up with the water changes, at least 1-2 gallons at least once every week. If any issues at all pop up do more water changes, not larger ones (even if you have to do them daily).
3-Plan your stocking carefully, you have very few options and no room for error, plan ahead.
Why do you want to go SW
? What fish or corals make you want to convert?