My Dad died recently, and I didn't speak at the funeral but later wished I had. I thought I wouldn't be able to cope with it, but actually at the time I would have been able to do it, so you will find the strength when you need it. The best readings are heartfelt and simple- maybe a lovely memory of something you did with your Grmadmother that you will always think of, perhaps a funny story from her life as a Grandmother or her childhood. Or you could read a poem that means something to you or her? Whatever you do will be lovely, you only have to start as though talking to a friend, something like "I wasn't sure how to start talking about Grandma, then I remembered the time we did x.." A friend of mine died suddenly in an accident and her FIL gave the most touching address at the memorial service, he just walked up and down the aisle of the church talking about her, about the funeral service a few weeks earlier, about her meeting his son. It was very simple and completely off the cuff but so human and touching. I really think if you just say what you are feeling then you really cannot go "wrong". Even just saying how much you loved her is enough. Good luck.
I did my nan's eulogy and it was well received. I just wrote down a load of my best memories and checked with the other gc. Then I took a theme from one of her favourite activities and worked it up into that. I was ok doing it until the very end when I finished with her way of wishing us good night. My voice cracked and I stepped backwards and stumbled off the dais. Don't do that!
So just start writing. Don't start at the beginning.