What is the subject
be sure to lock the door before going to band practice
a band
b practice
c door
d (you)

Each sentence is made up of 2 parts- subject/predicate (excluding simple/complex subjects and predicates [you'll learn that later on in 8th grade, and in high school] )
subject is a person place or thing (A NOUN).
a predicate describes what the noun is doing.
 The subject of this sentence is "the door".
The predicate of this sentence is "band practice