A Step-Up Transformer is one whose secondary voltage is greater than its primary voltage.  This kind of transformer "step up" the voltage applied to it.

How does a step Up transformer or step down transformer works:
A transformer is made from two or more coils  of insulated wire wound around a core made of iron. The number of times the wires are wrapped around the core ("turns") is very important and determines how the transformer changes the voltage. If the primary has fewer turns than the secondary, you have a step-up transformer that increase the voltages.  When voltage is applied to one coil (frequently called the Primary or input) it magnetizes the iron core, which induces a voltage in the other coil, (frequently called the secondary or output). The turns ratio of the two sets of windings determines the amount of voltage transformation. An example of this would be 100 turns on the primary and 50 turns on the secondary, a ratio of 2 to 1. transformer is nothing more than a voltage ratio device

With a step up transformer or step down transformer the voltage ratio between primary and secondary will mirror the "turn ratio" (except for single phase smaller than 1 kva which have compensated secondaries). A practical application of this 2 to 1 turns ratio would be a 480 to 240 voltage step down. 

A transformer is made from two coils, one on each side of a soft iron core. Step-up transformer increase the voltage.  A example if below. 

There are two points to remember

• Transformers only work with alternating current. Using direct current will create a magnetic field in the core but it will not be a changing magnetic field and so no voltage will be induced in the secondary coil.
   

• Using a step up transformer to increase the voltage does not give you something for nothing. As the voltage goes up, the current goes down by the same proportion. The power equation shows that the overall power remains the same.

P=V x I  Power = Voltage x Current

In reality, the power output is always less than the power input because the changing magnetic field in the core creates currents (called eddy currents) which heat the core.  This heat is then lost to the environment, it is wasted energy.

Electricity is first produced at the power plants. Electricity is then sent to step-up transformers where low-voltage electricity is changed to high voltage to facilitate the transfer of power from the power plant to the customer. Voltage must be increased so that the electric current has the "push" it needs to efficiently travel long distances.

From the step-up transformer, transmission lines carry the high voltage electric current long distances through thick wires mounted on tall towers that keep the transmission lines high above the ground. Insulators made of porcelain or polymers are used to prevent the electricity from leaving the transmission lines.

High-voltage transmission lines carry the electric current to substations where the voltage is lowered so it that can be distributed locally on smaller power lines known as distribution lines. Distribution line voltage levels are typically 4 kV or 12 kV. These voltages are reduced one last time at smaller pole-top transformers to utilization voltages, typically 120 and 240 volts, to make the power safe to use in our homes.