I don't really have experience with scripting in-game.
Script builders work using loadstring and a lot of other things. I don't recommend trying to create one until you're much better with Lua, because they (nowadays) have to be fairly complex to be secure.
I second what TheLuaWeaver has said, but here is some more information for you.
Give this wiki page a read through, and see if you can understand it all without too much difficulty. If you can, great! Make script builders to your heart's content. If not, look into what you don't understand more!
I'd also like to point out that there are few if any reasons to allow scripting inside of a game not dedicated to being a script builder, and even those have only limited use. Anything that need be done inside of a game should be doable without the need for a user to write their own code for it to happen.