hi vead
welcome ot the forums
OK to find the correct transformer ratings you need to do a little maths
ya dont get anything for nothing
the basic formula is ....
The DC output is approximately: 1.41 x VAC - (2 x 0.7)
That is after the bridge rectifier with the smoothing capacitor. But before any regulator chip if needed
Do you need good voltage regulation ? or is your smoothed 12V DC enough ?
if you need it well regulated then you will have to have a higher voltage out of the rectifier, cuz most regulators need at least 2V above the output regulated level.
So deciding what your DC voltage need to be work that formula backwards and determine what the secondary voltage of the transformer needs to be.
OK here's a basic regulated PSU capable of 12V and 2A make sure your transformer secondary is rated for at least 2.5 - 3 amps
give a bit more info as to requirements
cheers
Dave