Generally speaking you will find apple trees for sale at local nurseries in the spring so that is when and where you will find the most varieties.
However, don't just dig a hole and throw it into the ground thinking that it will be fine.
Be sure to keep the soil around the tree nice and moist throughout the summer as well as the spring.
Also, dig a hole that is twice as deep as the root ball and twice as wide then add manure, leaves (if you can get your hands on some (apple, maple, beech, Linden and so on) and some compost. Water the hole before putting your tree in.
Then fill with soil and manure/compost, watering as you fill in the hole.
Keep in mind the space that you have for your tree, if you have a lot of space and don't mind climbing a ladder to get your apples then a standard apple tree will work but, there are also Semi Standard, Semi dwarf and Dwarf varietes as well. You can also espalier your tree as well.
And keep in mind that you can graft several varieties of apple onto one tree.
As far as varieties: the world is your oyster. Try going to some local markets (especially if you have local farms) and try as many different types of apples as you can and choose one that you like.
I found a lovely un-named variety last summer from 1864 that was really nice and Tart. Royal Gala has to be one of my favourite varieties, John a Gold is really nice, granny smith is nice and crunchy and great for cooking, Gravensteens are a favourite amongst my family, my father has a spartan and Mcintosh and they both produce really nicely and taste nice as well.
Here's a neat website from Argifood Canada:
Some other sites to look at:
Also look at the bottom of this site to see what plants to plant around your tree to detur infestations, disease and help feed your apple tree.