To make a pizza you need meat and cheese, which first requires the upgraded kitchen. So if you don't have this yet, just keep playing and focus on accomplishing all the available quests to progress the game along and eventually the first NPC will give a quest to upgrade the kitchen.
Once you have done this you will have the Pizza as something you can create in the kitchen. Making cheese requires 2 milk I think, which can randomly drop when you kill wild boars, but to consistently get it you have to upgrade the farm to have a pen and capture some boars which turn into pigs. Once this happen you will get meat, fertilizer, and milk from your pigs. It's nuts but hey it's a game. The only other way to quickly get milk/cheese is to buy it from the traders once you build that building.
The ability to do this is really part of the "story" progression, but then I guess you have to CHOOSE pizza as the item you make, instead of say a cheeseburger.
Hope this helps...going from memory here so if I can make this better leave a comment and I can update.
Thanks!