I just started really playing & would like to build a base. Is there meant to not be flooring in your base? I also am having trouble building in general, I guess not fully understanding what to use and how. Any help with this would be appreciated!

You can build without a floor or there is grass and weeds stem floor.  You can put down a clay or pebble foundation if you like.    When building, things in place where you want them items will be blue.  When you look at the wall or whatever you built it will tell you what you need to build it. Get those items and go back to the wall or item you crafted and option to add those items should pop up.   Hope this helps if you have not already figured it out.  

You need to get recipes from npc in oak lab to build floor.