I just started learning Spanish, and I want to know what criteria I should employ when deciding if/when to begin reading short stories in my target language.
Well, the answer is: "It depends." When you have just started learning a language, most short stories will be too difficult, even stories that have been abridged and adapted for learners of Spanish (or whatever other language you might want to learn) (see graded readers).
However, in some cases, you may be able to read such stories if you are familiar with the writing system of the language you are learning and one of the following two conditions is true:
- If the language you are learning is very similar to your native language or another language you know well, you may be able to read very simple stories. For example, if you know French and you are learning Spanish, you may be able to understand some stories, especially if you are already familiar with the story's content.
- If you can find stories that are annotated with translations of individual words and phrases, or if the stories have a translation on the facing page, you may be able to read them.
These are rather specific cases. Generally speaking, extensive reading (or "free reading" or "reading for pleasure") to expand your vocabulary is a good idea. You'll notice soon enough whether the stories are easy enough or not.
(Personal anecdote: I started reading German chess books after only a few months of learning German (at one hour per week). Since I liked chess, it did not take long to learn German chess vocabulary, and the similarities between Dutch and German did the rest.)
I suggest you learn some grammar, understand the structures of the Spanish language and learn 200/300 core words.
If you hate studying languages with the usual methods, you can try Duolingo and use that, and then start reading stories etc.
I think you have to become A1/A2 to read books/watch films/read stories etc.
But, if you are Italian, French or Portuguese, you can try learning Spanish just by listening and reading. You'll understand it probably.