First of all the question is not right.You can not use a variable in programming like c,c++,java etc until it is not defined.Because if you wont define a variable data type, compiler could not recognize that and it will throw an error.So first define its data type and then use it.
from the values you assign to it p.s. if you use vb use this command as your first in your code option explicit this will force you to define the datatypes for all variables used by your program.