Remember, power of transformer must be the double of power in DC in your circuit.
Power Of Transformer = 2 * Icc * (Vsecundary-1.4)*1.4142
If current in DC is low (up to 300mA), you can use a full wave rectifier with middle point transformer. But in upper currents, is better to use a bridge rectifier.
DC voltage for a bridge rectifier is:
(Secundary voltage - 1.4) * 1.4142
For a 78xx regulator, this DC voltage is advisable to be 2v over output voltage of your source. For example, if you need 5V DC at output, secundary voltage of your transformer, must be over 7V or more.