They should be made of a material that will help wick moisture away from your feet. Some boots have smooth leather liners that mold nicely to your feet. Leather liners are harder to break in and can wear out faster than synthetics if you ignore their needs. You are going to be walking in those new boots for many hours a day in all sorts of weather and terrain. It's worth your while to get them fitted properly.
Timing is everything.
You should not get fitted for hiking boots in the morning, but in the late afternoon or evening after a full day's activity. Well, your feet swell during the day while you are in the more or less vertical position as a result of carrying you around and being part of whatever you are doing. This difference in size can easily make the difference between comfort and agony. Once you decide on the right time to go shopping, you are going to have to surmount the hurdle of finding a salesperson who actually is a competent boot-fitter. These folks are not common, and they are becoming rarer every day.