Adding an image texture can be as simple as going to the Material Properties tab (the little globe near the bottom - covered in this image), and selecting a new material (hit the plus button if there is no material already there). From here you can click on the input of the BaseColor, and choose Image texture. A box will appear allowing you to choose an image file from your PC.
If you need free textures, one good place to get them is https://3dtextures.me/
However, this site provides full PBR texture sets, which though look the best, are a bit more complicated to set up. this process can be made much easier by the help of a free addon. Go to user preferences, add-ons, and search for "node wrangler". Enable it.
From here, if you go to your shading tab, you can select the Principled BSDF and add whole PBR texture sets at once by pressing Ctrl + Shift + T, or a single image by pressing Ctrl + T.
Remember that your object must be UV unwrapped for materials to apply correctly. it works on the default cube because it comes with a pre-generated UV map.