Who is asking: Student
What you want for n in this case is the "size" of the matrix. A matrix is a square array of numbers, say with n rows and n columns. We usually say an nxn matrix.
You can start the induction at n = 1. A 1x1 matrix has only one entry, call it a. The determinant has value a and this is also the product of the diaginal entries. If it bothers you to think of a 1x1 matrix ten start the induction at n = 2. An upper triangular 2x2 matrix is of the form
Now try the inductive step.Penny