Start by hiring a great local agent who knows and regularly sells land in the area you are considering. There is just too much for a site like this to bring you up to speed on things.
Here are some considerations. Are there utilities on or near the land? Does the ground perk (for septic systems), what is the zoning and what is considered the highest and best use? Are there any easements on the property or are their easements on adjacent plots to provide access? Are there any wetlands or running water on the property? Is it in a flood zone? What type of financing will the seller provide? Do you have an alternative source of financing available if the seller won't provide any?
Decide upfront the purpose for the land and make sure you look for properties that are able to meet your intentions. These are just a few thoughts off the top of my head. Find a great local agent to help you. Best of luck.