On what page does Jem tell Dill how smart Scout is?

I believe you're referring to the episode when Scout and Jem first meet Dill. Dill introduces himself to the Finch children by his full name, Charles Baker Harris. For no apparent reason, he also says that he can read and that if the Finch children have anything that needs reading, he can do it.
Scout, for one, is not impressed, and Jem tells Dill that Scout's been reading since she was born. This is an exaggeration, of course, but Scout has been an avid reader for quite some time, and she hasn't even started school yet. Whereas Dill, despite what Jem describes as his puny appearance, is going on seven. So his proud boast about being able to read is really not that impressive when set against Scout's skills in that particular area.
There are a number of different editions of To Kill a Mockingbird. In the e-book edition that I have, the above action takes place on page 7. But whichever edition you use, it takes place very early on in chapter 1.