If you are a beginner Sildenafil 25mg is the right dose for you. In many cases, especially amoung young people with a psychological ED, 25mg of Sildenafil is sufficient to treat the ED problems. If you dont get the desired effect with 25mg you can increase the dose and take 50mg (on another day).
Dont forget to consult your doctor before taking Sildenafil. This is important.