The usual reference is "Categories for the working mathematician" by MacLane. I suggest you read this, but I'll give some other references. Check them out and pick what you like best:
"Arrows, Structures and Functors: a categorical imperative" by Arbib is a good book if you're not very familiar to mathematical structures. MacLanes book already assumes that you're familier to a lot of things (like abstract algebra, topology,...). If you're not, then this book is better
"Abstract and concrete categories" by Adamek, Herrlich, Strecker is the very best book on categories! Furthermore, it is freely available on the website katmat.math.uni-bremen.de/acc/acc.pdf The book provides a lot of theory, and a massive amount of examples (the examples are really important for somebody studying categories!). However, I would consider the book more of a reference text than a textbook. But I really suggest reading this book together with MacLane!