Well if the cruise is not a surprise for your Grandma I would sit down with her first and find out if there is anywhere in the world that she has always wanted to visit, if so you could look towards booking something in that destination. Secondly you need to look at what time of year you want to go. no use going to europe if its going to be bad weather.
Once you have decided where you want to go it is then about finding the right cruise for you, when booking my last cruise I had a lot of questions that i wanted answering so i searched around for the answers, after looking everywhere I finally came across a website whihc gave me all sorts of details about my cruise and what there was to do onboard. The website was [REMOVED], I ended up booking my cruise with the company, I didnt feel so happy about booking the cruise online so I rang their call centre where I booked my cruise with a very helpful member of staff called Tony, he helped me with all the questions that I needed answering right up until the date I left.
Hopefully this information will help you and you and your Grandma have a wonderful cruise :)