



 
Hi Maria, Mathematics is such a large subject and the areas of mathematics that interest different people are extremely diverse so it is difficult to advise someone else where to start. For me, when I was a teenager it was Euclid and a very good teacher that really got me interested in mathematics. A quote from Wikipedia helps explain why Euclid's Elements has been so important to many mathematicians.
In my opinion you can't go wrong starting with Euclid, PS: Euclid's Elements is available online for free (the copyright has long since expired). Click here.  


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