Grow at least two varieties, or three, for better pollination, and choose Hass as one of those, as it ripens much later than the others so you'll get fruit for longer.
Avocadoes are VERY easy to grow from seed - but you MAY need two varieties to cross-pollinate each other or you won't get much fruit, so it's best to stick to grafted varieties for your first two or three trees - then if, like me, you become an addict - or want a hedge - plant masses of seeds or as many different varieties as you can.
Alternatively, plant several sorts of seeds (you don't need to know the names - just make sure the avocadoes you got the seed from REALLY look different from each other) about half a metre apart from each other. Avocadoes grow happily in clusters - and as they are so close they'll dwarf each other, so your garden won't turn into an avocado plantation.
How to grow
Avocadoes will grow in tropical to cold areas- but trees in those cold areas MUST be sheltered from cold winds. Avocadoes also require perfect drainage, in full sun or semi-shade and hate ANY wind. Prune only to keep them manageable. MULCH THICKLY at least once a year, preferably with lucerne or with wattle tree slash (wattle branches, with lots of leaves, fed through a mulcher); feed with hen manure in spring. Cover with a hessian or shade cloth shelter for the first 2 - 3 years in hot summers or cold winters.
Trees can grow BIG - Wurtz is probably the smallest variety available, but even varieties that like to reach for the sky can be kept severely trimmed; in bad fruit fly areas don't plant thin skinned varieties. (Haas are pretty tough and are one of the smaller trees too.)
Avocadoes any size are edible - but the longer you leave them the better; different varieties and different climates crop at different times; if it comes off easily in your hand it's ripe. Small ones shrivel but still ripen eventually. Avocadoes don't soften till after they're picked. I leave some of ours on the tree for 18 months, long after the new crop is ready - and these elderly ones are SUPERB. (With luck - and if the birds don't get them - you may well have avocadoes all year round.)
There is rarely any need to store avocadoes - they keep best on the tree, for up to three months after the 'proper' picking time. We have been picking fruit from the Haas tree in the upper orchard for the last six months. If you must pick them or buy a 'special', keep them in the fridge, away from citrus fruit which will help ripen them. If you buy them hard and green and want to ripen them, stick them in a paper bag with a piece of citrus.