I agree with the above. You need to pick one major topic the book covers and make it more specific. The link below is a helpful thread outlining how to turn a "subject" into a theme statement.
This now leaves the question: "What subject should I cover." I encourage you to brainstorm one or two major subjects the book talks about and some plot details that would support each. This way you know you are dealing with a subject you have KNOWLEDGE of. Examples from Huck Finn inlude (obviously) slavery, childhood, life lessons (and whether they are learned best by instruction or experience), friendship, honesty (this could be fun considering how often Huck lies), or adventure.
The most important consideration in writing a thesis statement of this lengthy novel is to limit yourself to an idea that can be covered in detail in an essay, for too often students tackle too broad a topic and the result is a superficial examination of this topic. So, with this having been said, you may wish to read some criticisms written by professional writers which may help generate specific ideas for you. Also, you may wish to consider one of the basic elements of the novel such as theme, or character, or point of view, or setting.
For instance, the settings of the river and the land offer interesting comments on society by Twain as on the river Huck and Jim are just two human beings who are equals, friends. But, on land, Huck must make the pretence of owning Jim and treat him according to the dictates of this relationship.
Another element that you may wish to consider is Twain's wonderful satire. On a link which is listed below there is an outline for an essay on how Twain satirizes religion, education, and slavery. Also, there are outlines for other topics. So, please take advantage of these aids that eNotes provides.