Go to your local shop that sells high quality backpacking gear. Try on lots of boots to see which ones feel good. Different brands have different shapes and fits. Once you find one that feels comfortable, have them fit the boots to you. If you feel pressure or some part of the boot poking you, don't uy it. Make sure you wear the same wood socks and liners that you'll were.

There should be very little movement. Also, try to push your foot far forward inside them (sometimes you can "hang" your foot off of a step) - your toes should not touch the front of the boot.

Though nylon boots with integral Gortex socks are popular, I've read a lot of complaints that they will eventually leak. I tend to prefer 100% leather boots with a good waterproofing treatment.

For gators, go with Gortex.