1. Socks: A good boot is practically worthless without quality socks to go with it. You are looking for something fairly heavy with attention paid to the heel and the ball of your foot. Most places that sell boots (reputable shoe stores, ect) will be able to help you find a sock that will work well with your boots. I use some that I bought at Tractor Supply Company. They are a lighter sock for summer, but have a heavy toe/heel area with added elastic for arch support. I also have some n-genious thin-sulate socks I bought from Dick's Sporting Goods for cold weather use. I wear the TSC socks on a daily basis to work, where I stand for 7+hrs. If I wear a normal pair of socks with my boots, my feet are about to die by lunch time.
2. Fitment: If this is your first boot purchase, then buy your socks and boots at the same time to insure they work together. With your new socks on, slip your foot into the boot. Check toe room. The worst thing I've ever experienced is a pair of boots that were too tight around the toe. Next, take a step in them. Your heel should move up and down very slightly (~1/2") in the back of the boot when you step. This ensures that the boot will not rub your heel raw (sounds counter-intuitive, huh?) when wearing them. If you're getting steel toe boots, make sure they are spacious enough around your little toe and large toe to prevent callouses from forming. They make boots in several widths, as with tennis shoes, and most places won't think twice about ordering you a set to fit your needs.
3. Styles: There are different styles of boots, obviously. You have to decide what you like with this. I prefer a more work oriented boot. The cowboy boots just don't fit my feet and the large heels inevitably cause me to sprain an ankle. I also prefer slip on boots such as the Red Wing Pecos line. I have a pair in steel toe low top I wear for farm work and off-roading. The boots I wear at work are a Wolverine lace up. They have padded soles, heavy insoles, and a large steel toe, as well as provide amazing ankle support for long periods of standing. Look at spending at least $100 on a pair of boots. My Red Wings are almost 5 years old, and still taking a beating.
4. Break-in and care: For break in, you are going to have to wear them often. A boot is not comfortable until they are broken in. It usually takes a few weeks to get them totally broken in, but afterward they will work great. As for care, I try to polish my boots once a month with Meltonian Boot and Shoe Cream Polish. It keeps the leather conditioned and helps prevent cracking. I also treat them with mink oil about every three months to keep them water proofed.
I hope this helped some of you. Again, this was taken from my experience with boots and is the advice I give to people when asked. Happy prepping! :D