It has been awhile since I last visited
the link provided by
@davenn but, having looked again, I must agree it is an excellent explanation of transformer operation.
Part 1 will generally provide all the information you need to specify and use any power transformer.
If you want more details, Part 2 provides that too.
There are no fancy maths involved, just simple algebra. Highly recommended for beginners, and useful for review of principles even if you are an expert.. Highly recommended!