There's honestly a ton of ways to do this. The most straight-forward would be to just create a script file that defines the properties of each item, though, its also least flexible in that it makes it difficult to change things after you've shipped.
A data-driven approach is probably preferable, like Derek suggests; you can use JSON or some other format to your liking. Load and parse them at run-time so that you can add or change the items later if you need. There's a way to specifiy that an arbitrary file is meant to be published with your Unity game package, rather than processed at build-time. You might want to do some basic hiding/obfuscation of any plain-text files, just to deter the casual cheater, one easy way is to put these things in a .zip file, or to do some basic XOR "encryption" -- but don't get caught up trying to come up with a perfect anti-cheat thing because you won't (people much smarter than you and I haven't yet in their entire careers, you or I aren't going to do it in our spare time) all you want is to prevent people discovering easy cheat attacks while casually browsing your game files.
More generally, the tendency with Unity, especially if you're less experienced, is to think of everything in the game as *the* thing -- you have a game with 12 loaves of bread, and you literally have 12 loaves of bread defined inside Unity, and maybe you help yourself out with prefabs. But what your really want in this instance is not to define disticnt game-objects for each instance, but to be able to create instances from a kind of recipe. What you store in the JSON (or whatever) file, is the recipe.
