I'm thankful your grandmother gave you a Bible for Christmas; no gift could be more valuable. Why is that? The reason is because the Bible tells us who God is and what He has done for us, and also how He wants us to live. Can you think of anything that's more important for us to know?
At the same time, many people have the same trouble you've had reading the Bible, and it can be difficult to understand if you don't know how to go about it. It's helpful to remember that the Bible actually is like a library—a collection of books (some long, some brief), written under the inspiration of God by many authors over hundreds of years.
But just as you wouldn't walk into a library for the first time and start by reading the very first book on the shelf, so you won't necessarily want to start by reading the first book in the Bible. Instead of starting at the beginning, therefore (like you do with most books), I suggest instead you start with what I call the Bible's "center": the four Gospels. (I often suggest people begin with the Gospel of John.)
The Gospels tell us about the life, death and resurrection of Jesus—and once you understand who He is and what He has done for you, the rest of the Bible will begin to make sense. Make the Bible part of your life every day, because "Your word is a lamp to my feet and a light for my path" (Psalm 119:105).