I'm currently only on 1st semester Math Undergrad, but I'm quite interested on the topics of Ring Theory, even though there's no subject for it yet on the 1st semester program
I would recommend the following book on abstract algebra:
A book of Abstract Algebra
Author : Charles C. Pinter
In my opinion, this book is excellent for getting initiated into basic and serious study of abstract algebraic structures, such as Rings and Integral domains.
I would certainly recommend A Book of Abstract Algebra by Charles Pinter. After that one, you might tackle An Introduction to Ring Theory by Paul Cohn but it does move at a fast rate and gets into some pretty abstract topics early on.
For commutative rings (which are the most relevant to number theory) I would recommend Irving Kaplansky's "Commutative Rings", but you would need to read a basic primer on abstract algebra first.