When a d.c voltage source is applied across the primary of the transformer,the current in the primary coil remains constant. Hence there is no change in the magnetic flux linked with the secondary. Therefore the voltage across the secondary coil is zero. Thus a transformer can't step up dc voltage.

Transformers only work on alternating current (a.c.). The current in the primary coil causes it to become an electromagnet. The continually changing current produces a continually changing magnetic field in an iron core.

Transformers only work for alternating current, because you need a changing magnetic field. The size of the induced voltage depends on the number of turns on each coil – if there are less in the secondary coil, the voltage falls, and if there are more in the secondary coil, the voltage rises.