explain charging by friction, conduction and induction.

In using friction it doesn't matter whether the objects are charged or not but it would be usual to start with uncharged objects. 
All that is required is that the materials have different affinities for electrons so the forces will pull electrons from one material to the other. 
If you were seeking a negative charge and both objects started negatively then it would assist your particular goal. i.e the item to become negative would have a greater charge than if you started with neutral objects. ( the other item might become neutral or positive but we aren't concerned about it). 

For conduction it is necessary that at least one has a charge. This charge is distributed over both objects. If they both had charges of the same sign but different magnitudes the charges would still distribute. 
If they had opposite charges then the two could "cancel" each other out leaving no net charge. 

For induction at least one item must have a charge. And there must be at least three objects involved. The original charge source is used to SEPARATE the other charges into two parts hence there must be two other items to each accept one part of the induced charge. 
( often we use the earth but that is still an object) 

The two items that were to accept charges by induction may already be charged without altering the effects. Naturally if they had different charges then the effects of conduction would partially cancel each other and lessen the charging by induction.

charging by friction:-we have seen that some objects can be charged by rubbing against another object. when two objects are rubbing together, some one objects get transferred to the other objects that lose electrons become positively charged and those that gain the electrons become negatively charged. this method of charging an object by rubbing it with another object is called charging by friction...