Depending on what washing machine you have, it can be very easy to remove the agitator.
First you must determine the brand of washing machine you have.
To remove agitator on a whirlpool washer, remove top cap from agitator, Next you will find a translucent cap, remove this with your pliers. Then you will see a 3/8 inch nut, Back this nut out and you can then remove the agitator. Be Careful though, inside the agitator where the bolt has been removed is a white piece of plastic that holds the "dog ears". These dog ears are what lets the agitator move in one direction and locks when the transmission moves back in the other direction.
Maytag and Admiral washers are very similar to remove. General Electric, (GE) is different, To remove a GE agitator simply pull upwards very hard, GE agitators snap on. Be careful not to pop it into your mouth, that really hurts.
Remove agitator, repair or replace and reset, GE just push on, you will be able to see the groves to line up the agitator to the hub. For whirlpool place bolt into dog ear guide, and set down on top of transmission post and tighten, be careful that dog ears, do not get out of alignment. That's it.