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Elo fide

I'm thinking about joining a federation but it seems I can't join fide directly. First I must register and play with the national elo (in Spain it is FEDA elo, in the US it is USCF elo I believe). When could I sign up for fide and thus be able to have an international rating?
Once you sign up for the National Elo of your country. Have either a USCF membership or Spain Membership, then you can get your own fide ID. There's more steps that require filling out a form, look it up... you'll find the rest.
Most people sign up in a local chess club, and they register you at the national organization, and also at FIDE. This may make sense if you want to play OTB anyway, as some tournaments might be restricted to members.

Though it may be possible to get a FIDE-ID without being member in a national organization, directly or indirectly. (For example in Germany, you would still apply through "Deutscher Schachbund", the German head organization, even if not a member in a German chess club).

I think you can even get a FIDE ID through their Arena (Pro) stuff online, but I don't think many people go this route.