The difference between dwarf / semi-dwarf / and full will make a huge difference. Most apple trees are grafted onto a different root-stock. If you buy from a nursery, it'll be grafted on "EMLA 7" most likely. That's a highly disease resistant semi-dwarf root stock. If you bought a full tree or as bare-root, there is probably no graft, it's just the tree's natural root. If you buy it from an orchard or an orchard supplier, it'll probably be grafted on "M9" stock. M9 is a dwarf stock, that roots very deeply, but doesn't spread wide, making it ideal for high density planting (a 3 to 4 foot spacing). Dwarf are typically 3-4 years. Semi-dwarf 3-5 years, and full depend more on the variety in the absence of another root stock, but 5 years is common. You'll get fruit before then, but 10-20 per tree at most, and they tend to be small and drop (or get robbed by deer and squirrels).
Apple trees also need lots of Calcium and Magnesium to fruit well. I recommend if you have chickens, spread their oyster shells or other calcium source under the apple trees. The trees will use the excess, and the chickens will fertalize the trees. Otherwise, you can just buy the calcium granulas and apply once every 3-4 years. As for magnesium, Epsom salts in a sprayer, set to 1 tsp per gallon, one week before you would apply the oil spray (when buds are just forming).
A pollinator is a necessity. Even if the tree you plant is self-fertile, another pollinator tree can easily double your yeild. Different apple trees bloom at different times, so make sure you get a variety that blooms at the same time as your other tree. Plant as close togeather as you can. People worry about overcrowding, but they shouldn't. 1200 trees on m9 stock per acre is ideal. If you're only planting 2 or 3, or have a different root stock here's the rule:
Dwarf: 3-4' spacing
Semi-dwarf: 5-6' spacing
Full: 8-10' spacing
If planting rows, that's the spacing within the row. The spacing between rows should always be about about 3 times the plant spacing with a minimum of 12' to allow you to walk between them for harvesting and to allow more light to penetrate the back rows.
Pruning is the next most vital aspect. Trees fruit on second year growth. You'll see little spikes or mini-branches about 2-3" long portruding from the main branches. These "spurts" are where the fruit will form. Don't remove them. However, old wood doesn't produce them, so if you have branches that are 3 years old, or that cross the center of the tree, cut them off. Here's a very basic tutorial I found. http://www.weekendgardener.net/how-to/prune-apple-trees.htm
Mulit-variety and multi-species grafts are popular. I recommend against it. Yes, you can grow a dozen types of apple on one tree, and even grow apples and pears and plums on the same tree. But the grafted branches grow at different rates and are susceptible to different diseases. In the short term (10-15 years) they're nice to have, but quickly become a lot of work and are very vulnerable to rust, scab, parasites and pests. On such trees, you don't get all of the strengths of each variety, but you do get all of their weaknesses. The only reason to go that route is if you have limited space, but with dwarf trees and proper spacing (which most people seriously overestimate), you can plant 4 trees in the area most people would only plant one, and get better production and healthier trees.