The problem is you either haven't closed your if or you need an elseif:
create procedure checando(
in nombrecillo varchar(30),
in contrilla varchar(30),
out resultado int)
begin
if exists (select * from compas where nombre = nombrecillo and contrasenia = contrilla) then
set resultado = 0;
elseif exists (select * from compas where nombre = nombrecillo) then
set resultado = -1;
else
set resultado = -2;
end if;
end;