First time flight to Reagan National Airport ?

I am from Spain, I'll be traveling to Washington DC for the first time from JFK-New York, so I had a few questions.
When I first get off of the plane, where do I go? Is the first place Customs, or do I get my luggage first? At the Customs section, what exactly happens? Will my luggage be searched, wich questions the official migration will de asking me, do they speak English and Spanish?

Thanks a lot!

As others have said, you will go through customs in New York. They have interpreters, but it will be faster if you can answer the fairly simple questions you have in English (your English seems pretty good).

They will ask you where you are going, where you are staying, what the reason for your trip is (business or pleasure (holiday)). They may ask to see a return air ticket. They may ask for a US Address (usually the name of the hotel is OK). You will be photographed and fingerprinted.

As others have said, then you pick up your bag. Your bag may be searched randomly at any point on the journey by the TSA. That is standard for anyone. They may search your bag at customs if they have any concerns. They can take you for more questioning. Both of these last are rare. It may happen, though, if you are coming for business, since this is only permissible in a couple of limited circumstances.

This all could take an hour, if the immigration line is slow. You will then be able to return your luggage to the system (look for a place to drop luggage for transferring passengers just after customs.) and you go on to your next flight, which is now a domestic flight to Washington. For that flight you only need a photo ID. (your passport should do nicely).

You will go through customs in JFK-NY.

You usually pass through passport control, get your luggage, and then go through customs. You get a customs form that you fill out while on the plane.

If you have something funny on your form, then might ask to see inside your luggage. Otherwise, you pass right through.

[edit]
I'm not sure, but they will probably have a Spanish interpreter, since it is somewhat a common language in the U.S.

[edit2]

There are signs (in English) when you get off the plane, and there is only one way to go, at either airport, so you cannot get lost. When you switch plans at JFK, you might need to go to another terminal to get your connecting flight (after you pass through customs).

In Reagan-National, the luggage pick-up is downstairs if you are in the new terminal. If you get off the old terminal, luggage pick-up is by the door as you go out. They have signs, so it is hard to miss.

You'll clear customs at JFK....you'll be asked basic questions (why are you here?, did you pack your bag?, etc.)

Your bag(s) probably will be searched

and you should have no trouble at either airport with language

First there is no customs section at National Airport as it is not an international airport. You will go through customs in New York. When you get off the plane you will go down to the first level, that is where the baggage claim is located. Just look at the monitors to see where the baggage claim from New York is located.