Greek is the language of the New Testament, and if New Testament Greek is all you're planning on working with, your journey won't be terrible. There are only so many words -- it's not like you need to learn the words for carburetor, pizza, and keyboard.
The Greek of John is considered "baby Greek". That is, it's easy. In reality, the whole of New Testament Greek is considered easy to those who are comfortable with the works of Homer. If you don't know Greek at all, this should encourage you to accept that Koine Greek, the Greek of the New Testament, isn't impossible.
Basics of Biblical Greek Grammar ("BBG"), 2nd edition (Bill Mounce)
This is not the greatest Greek text. It's just very helpful to use as a guide throughout your studies. The author's tips are helpful -- especially his tips on what NOT to do while doing your first-year studies. His supplemental materials on his website are also good. That said, you want the 2nd edition. The 3rd was too large. You can get the 2nd edition for $5 or so. The one major negative is that the author's understanding of the Greek verb is entirely broken. You'll get far with working with Greek, but not with understanding nuance. The exegetical insights at the start of each chapter make up for a lot of flaws. You should get the book if only for those.
Basics of Biblical Greek Grammar, Workbook
The workbook is fantastic. I buy them in bulk when I see them in Mardel clearance. I go through them every few years to keep my skills polished. You can find the answer key online. Workbooks without answer keys are pointless. You should try to get used if its not marked.
Reading Koine Greek: An Introduction and Integrated Workbook (Rod Decker)
The content of this book is much more accurate than BBG, but the "integrated workbook" is quite anemic.
An Introduction to Biblical Greek: Elementary Syntax and Linguistics (Dana Harris)
I just started working through this one, so I can't review yet, but, when the author gets to verbs, verbal aspect comes up right away. This is critical. There's also a workbook.
A Reader's Greek New Testament
In BBG, the author tries to get your confidence up by telling you that 80% of the NT is just 300 words. In reality, that means you're looking up every 5th word. Not helpful. However, the Reader's Greek New Testament puts a gloss of the Greek words at the bottom of the page. To use this text, you do need to understand the basics of Greek and a solid vocabulary. Those 80% of those words won't be defined on the page -- that's on you.
Greek Grammar Beyond the Basics (Daniel Wallace)
Most people generally consider this book to go with BBG -- though it's not formally related. After the first few chapters, I generally use it as a reference.
Daniel Wallace has one of the wildest Greek stories I've ever read, you can read it in Lee Strobel's The Case for the Real Jesus. It's wild.
Intermediate Greek Grammar: Syntax for Students of the New Testament
While not formally linked, this book generally follows the Decker text. You really do need multiple intermediate texts to get an idea of the various ways you can think about the nuances.
Basics of Verbal Aspect in Biblical Greek
One of the wild things about Greek to an English speaker is the importance of verbal aspect (the part of the language BBG gets wrong). This book is important to help you avoid pitfalls. It's critical if you use BBG as your primary text.
Morphology of Biblical Greek
This is an interesting text for hyper-geeks. If you're curious about the relationship between words, or want a better way to remember an odd form, this book is for you.