First you have to have a toilet plunger. That is one that has a bulb shape, or a folded skirt inside a flat plunger. The round shape or skirt is needed to seal to the sides of the toilet bowl. A flat plunger just won't do the job.
Then push down gently so as to not squirt everything all over the room.
Then pull up with energy.
The real un-blocking action is to suck the blockage back into the toilet, loosening it all up and allowing it to then flow on through. Trying to force it into the drain can make the block worse, or even force a drain pipe open.
For an animation on the power of pulling rather than pushing on clogs, click on "IT'S THE UP-STROKE THAT COUNTS".