doesn't ohm's law applied to every thing?
doesn't ohm's law applied to every thing?


No, Ohm's law describes ohmic devices only.  From the Wikipedia article "Ohm's law":


  An element (resistor or conductor) that behaves according to Ohm's law
  over some operating range is referred to as an ohmic device


But there are many devices that are not ohmic such as capacitors, inductors, and PN junctions to name just a few.  You will learn about these later in your studies.


  I've read that voltage is high in step up transformer due to greater
  no. of turns it makes sense but why it reduce current?


A transformer is not an amplifier; it cannot increase the power.  Ideally, there is no power loss (practically, there must be some loss).

But power is the product of the voltage and associated current.

$$p = v \cdot i $$

Thus, if the secondary voltage is greater than the primary voltage, the secondary current must be less than the primary current in precisely the right proportion such that the primary and secondary powers are equal:

$$v_P \cdot i_P = v_S \cdot i_S$$

or

$$\frac{v_S}{v_P} = \frac{i_P}{i_S}$$

For example, if the secondary voltage is twice the primary voltage, the secondary current is half the primary current.