Wheat is a hugely important item in Genshin Impact when it comes to cooking, and you will need it to make some of the games more complex recipes. You do not use wheat itself when cooking, instead you process it into flour which can then be used to make food.
Food is vitally important as it will give you powerful buffs, and also heal you from any damage you take in battles. Wheat can also be quite difficult to find in the early game as it will just randomly drop while scavenging, but fortunately, there is a spot in the early hours of the game that will yield plenty of wheat.
In Springvale, to the west of Windrise and the south of Mondstadt, you will find a dangerous area called Drunken Gorge. This used to be a busy shopping area, but variously monsters and creatures ended up setting up camp here and taking over.
If you head into the gorge and search all the crates, barrels, and old merchant carts, you will find plenty of wheat. Once you have the wheat, you will need to turn it into flour, and to do this you will need a cooking pot that will be marked by a spoon and fork symbol on the minimap. You can actually find a small cooking fire right that the Springvale end of the gorge.
Interact with the cooking fire and then switch to the second tab, which is called Process. Click on the wheat in the list of ingredients and you will be able to process it into flour. All ingredients have a timer for processing, which could be up to a minute or more. You can queue up lots of items for processing, then go and play a different part of the game, and return later to collect all your processed ingredients. Just interact with the cooking fire, go to Process, and then hit the prompted button to obtain your processed ingredients.