Typically the convention is to introduce a new scope with let. You can, if you really want to, declare the names at the top and then assign values later, similar to old C, though this is not idiomatic:

  (let (foo bar baz)
    (setf foo 2)
    (other-code)
    (setf bar (expt foo 2))
    (more-code)
    (setf baz 5))