When you bin an avocado seed, you are wasting a potential plant for your greenhouse, conservatory, if you have one or for the local charity sale. The seed of the avocado grows "true to type" i.e. it is a clone of the parent plant. Basically, it is a free seed.
You may want to buy an organic avocado if you can find one. First, eat and enjoy your avocado from the supermarket and retain the seed. Stock up with coir (coconut compost) which is crumbly and nutritious and allows water to reach all the roots. Also get a couple of clean plastic pots. Soak your seeds in water for 30 mins between 40-50 degrees centigrade to prevent root rot.
Prepare your pots and water the coir well:
Take a carving knife and cut off the point end of the avocado seed as shown. Take care, as they are slippery!
Place seed into each pot so that the cut top of the seed is level with top of the coir:
Don't expect too much in terms of more fruit but the plant should be quite nice. It was free, too!