Hi,
A frequently asked question...therefore you may find many useful threads here in this forum.
Generally
* start your design with deciding your specifcations (don't choose a transformer at first)
* then decide useful circuit topology
* then choose SMPS IC
* then choose the periferal (passive) devices...like the transformer.
My hint:
If you look for
* latest datasheets
* application notes
* and other help for your design (like simulation software, selection guides...)
...then go to the IC manufacturer's internet site.
Klaus