Avocado trees make great houseplants and are easy to grow from seed. You won't get any fruit but they're really attractive plants.
The first thing you need to do is remove the large seed from a ripe avocado. Remember that the roots grow down from the fat end of the seed and the stem will grow from the pointed end.
There are a couple of ways to grow avocado from seed. Some people like to immerse the seed in a glass of water. The seed is supported at the top of the glass by toothpicks pushed into the top 3rd of the seed. The seed sits in the glass, half covered with water. Eventually roots will begin to grow down into the water and a stem will begin to grow. The plant is then carefully transplanted to a pot.
That's a great way to see how the roots and stem of a plant begin to develop, but I think the plant will be healthier if the seed is planted directly into a pot. This way there's no risk of damaging the roots when transplanting from the glass to a pot.
Half fill a 120 to 150mm high pot with old compost or a good organic potting mix and pat down gently. Place the seed on top of the compost, fat end down. Fill the pot with compost and give it a good soak. Place the pot in a warm spot but not in direct sunlight, perhaps in your bedroom near a window. Keep the compost moist but not soggy. Within 4 to 6 weeks the stem should have appeared.
Once the plant is well established, perhaps after a year or so, pinch out the growing tip. More stems will grow from this point and the plant will become much bushier.
Keep transplanting into bigger pots as the plant grows bigger.
Use whichever of the 2 growing methods you prefer. If you have more than 1 seed, try both methods.
NOTE: Sometimes seeds don't germinate. Don't give up. Try again with new seeds.