Introductory number theory is relatively easy. When I took it we covered primes, quadratic reciprocity, algebraic numbers, and lots of examples and relatively easy theorems. Most of the proofs we did in the class were very straightforward (wilsons & fermat's little theorem, etc) and was not difficult at all. The 'next level' of number theory, Algebraic number theory, involves upper level algebra and can be difficult at first glance, but if you have done any studying in field theory or a related subject you will recognize some stuff.
Number theory may not seem like the most practical thing to learn but it gets used in group theory, discrete math, and other typical third year math courses.