Naming a tortoise is much like naming anybody else.
First, I would watch the tortoise very closely. What is this tortoise like? That might give you an idea.
Second, I would research the type of tortoise I needed to name. Where if your tortoise from? Is there a word in the language of the tortoise's natural home that would make a good name?
Third, I would think about names that I like. For example, I might want to name a tortoise after a family member or a famous person.
If you listen very, very closely, your tortoise may just whisper a name in your ear.
When you have a name for your tortoise, I hope you will post it in the brag book. You could tell how you decided to name it.
PS I got my name because of my shell. That's not a unique idea, but I like it.