Isn't there a picture and specs on the back of the package or inside under the bubble? The three yellow wires should be the secondary and two will be the "outer" legs while the third one (usually located between the other two and often with a stripe or different color altogether) will be the center tap of the secondary. There are a variety of way you can use these three leads, depending upon the voltage you need. Across the two "outer leg" leads, you should have the 25.2 volts. Between the center tap and either outer leg, you should have half that voltage. You can use a bridge rectifier with the two outer legs as the input for the highest full-wave rectified DC output voltage. You can use a single diode with these two legs as the input for the highest half-wave DC output. All three wire can be used with two diodes for fullwave DC output with half the voltage as the first option. A bridge rectifier can be used as the input on a bridge as with the first option but with the center tap grounded so that the "+" output of the bridge gives you a positive polarity full wave voltage at half the level of the first option and the "-" output of the bridge gives you a negative polarity full wave voltage at half the level of the first option. And there's even more. A good electronics reference book on rectifiers and filters would be helpful.
Dean