First, you will need to meet Preston Garvey and his band of survivors before you start crafting. In Sanctuary, you will get a quest from a man called Sturges, who will provide you with the basis of the crafting thing. This will allow you to build beds and other structures, dealing with water, food and establishing defenses. Once you learn the basics, get back to the Workshop menu and look at the bar at the top of the screen, that will tell you in synthesis the resources which you need to build useful items. You can "tag for search" certain items so you know, while traveling along your trip into the Wasteland. You can even scrap items in your own settlement to craft stuff you need. Two things you will need to remember as your settlement arise and grows in size: you will have to deal with power generators close to each item you will place and needs power to work; people who inhabit your refuge can also guard spots and artillery cannons, so they are an actual resource you can use at your favor.