The Standard Spanish comes from Castilla-La Mancha. Central Spain. but it is also spoken in all Spain. Spanish has different dialects. For me, I would say, the most difficult for a foreigner is the dialect spoken in Andalucia, South-Spain. In addition in Spain we have other languages, So in some places we are bilingual speakers. I am from Catalonia, where Barcelona is placed, we speak Spanish and Catalan. In Galicia, they speak Spanish and Galician. In Asturias, they speak Spanish and Asturian. In País Vasco, they speak Spanish and Euskera. You can go anyplace in Spain to learn Spanish. Dialects mean they have different accents and maybe small differences in some expressions but you'll be understood all over Spain. Hope this helps! Have a nice day!

There is no such a thing like "Standard Spanish", maybe in Spain, but not generally speaking.

Well, firstly, if you want to study spanish, you must think about why do want to study it, or in what region you are interested in. Spain or any country of latin america, Most of the differences that you will find are about pronounciation. I would say that Spanish from Spain is more difficult, they speak a bit faster than us. But at the end, you can choose any spanish dialect, people will understand you. any way. Just think about if you want to learn spanish from spain or from latin america. both of them are ok for a beginner.

As a general rule, spanish speakers can understand others dialects. I can easily catch spanish from spain or spanish from colombia, that's no problem, there could be some words I don't get, but in a formal conversation, those words aren't usually used. The Spanish Royal Academy (Real Academia Española, or RAE), is the academy that gathers all the information over the language. You should reffer to it for doubts.

I would like to say that all the spanish dialects are good to start with, it depends where you would like to go or which culture you like the most, but the standard one is the spanish from Spain.