Funnily enough we would suggest that the first consideration when buying an apple tree is location. Where you plant it impacts culture, size and variety.
In an orchard setting you are likely to have free standing trees. Typically these are bushes or half standards. Bushes are easier to pick, half standards are easier to mow under. If you have some room, then you get more fruit from trees on a semi-vigorous rootstock such as MM106. With less space use M26 and with only a little opt for M9 rootstocks (although these will need permanent staking). There is considerable rootstock choice throughout our range of apple trees.
In smaller spaces, the proportion of vertical to horizontal surfaces increase. It then begins to make more sense to grow trees trained against walls, fences or straining wires and you should be thinking about cordons and espaliers. Cordons are generally on M9 rootstocks and espaliers are on MM106. Remember that both carry far less fruit than free standing trees, but they also take up far less space. Cordons can be planted as close as 60 cms apart in a row which means you can have 6 varieties in less than 5 metres. Espaliers take more space needing at least 2 metres each, but they also carry larger crops than cordons.
In the smallest areas such as patios, balconies, decks and terraces without beds, apple trees will need to be grown in containers. Larger containers of 30 litres and upwards will provide a decent home for a bush on and M9 rootstock. Otherwise you would be well advised to look at ballerina apples such as Flamenco and Samba which will live happily in smaller pots. Remember large pots filled with wet soil based composts (which should always be used) are heavy so be careful with loadings on roof gardens.
Aspect affects all apple trees. Whatever you grow is hugely impacted by the aspect of the planting position. South-west is best, but it can be windy. East is worst if it is in full sun early in the morning as the warmth of the sun destroys frozen flowers before they have defrosted normally (so late flowering varieties would be better here). All fruit needs light and warmth to ripen, so in colder spots, think about planting against a wall that will retain and reflect heat to help ripening. Don't plant frost prone varieties in frost pockets. Try to plant earlier cropping varieties - which are also later flowering - as you go further north. Altitude is not good - the higher you get the harder it is for pollinating insects to do their job - the air is thinner and the winds are stronger. Take extra advice if you are planting at more than 650 feet above sea level.
Where you are geographically defines where it is best to grow apples in your garden. Where you grow apples in your garden defines the rootstocks you should use and the shapes in which it is possible to grow your trees.
This is the easy and the fun bit. There are over 4,000 varieties of apple tree and one person's meat is another's poison, so there is little point in recommending specifics. However, here are a few general pointers. Early ripening apples tend to have thin skins. This means they do not store well and so are less often found in supermarkets. They are delicious off the tree however. Some apples store and ripen better after picking than others. These are the ones that you DO find in the supermarkets and it often makes sense not to grow them as when in season they are plentiful and cheap. Having said which a Golden Delicious off the tree is incomparably better than the things you find on high street shelves.
Many older varieties which are generally only known as cookers also turn into eaters if you leave them long enough. They tend to have longer cropping seasons more modern varieties and the later fruit are sweeter. Howgate Wonder (which is a huge cooker) is utterly delicious eaten raw late in the season and by Christmas it has a seasonal cinnamon taste.
If you only have room for a tree or two, look to the multi-purpose varieties such as James Grieve which is eater, cooker and juicer rolled up into one.
If you are growing espaliers and cordons (in particular) you really can have enormous variety in a small space. If you do that, make sure that you choose your trees so they cover the cropping calendar otherwise you will have a glut one month and nothing to eat the next.
Apple tree pollination is subject that vexes many people. Don't let it. We have a special pollination advice page, which you should look at if you are planting an orchard, if your garden is well away from other apple trees or if you are planting either very early (Group A) or very late (Group H) apples. Otherwise, there will certainly be an apple tree or crab apple near enough for bees to do one of the most important jobs on the planet and pollinate your apples trees for you.