> I'm not sure where I should start? Is there a beginners guide for SQL, relational databases in general or anything else I should read/learn?
>

There�s many vary much plus more SQL tutorials ;p
Any that did not escape from SQL standart may help you.
For simple reference take a look in these:
http://www.w3schools.com/sql/default.asp
http://sqlzoo.net/

for reference on Firebird check Interbase 6.0 SQL reference and "Firebird 2.0 Language Reference Update" in:
http://www.firebirdsql.org/index.php?op=doc

and a explanation on joins types:
http://www.gplivna.eu/papers/sql_join_types.htm

for any SQL theme you can verify wikipedia too... in ex.: verify "Relational database" in http://en.wikipedia.org/wiki/Relational_databases or "Database theory" in http://en.wikipedia.org/wiki/Category:Database_theory

I think these links should help you understand SQL and databases in general like you asked. But agree with others, for understand the FireBird-RDBMS you can read FB Book.