Q:

Create Vector of Length n in Julia

I want to create a vector/array of length n to be filled afterwards. 
How can I do that? 
And does it have to be filled already with something? 

A:

For example if you want a Vector of Ints of length 10 you can write
v = Vector{Int}(undef, 10)

And more general for an Array of Ints of dimensions (2, 3, 4)
a = Array{Int}(undef, (2, 3, 4))

Note that this fills the Vector/Array with garbage values, so this can be a bit dangerous.  As an alternative you can use
v = Vector{Int}()
sizehint!(v, 10)
push!(v, 1) # add a one to the end of the Vector
append!(v, (2, 3, 4, 5, 6, 7, 8, 9, 10)) # add values 2 to 9 to the end of the vector

sizehint!  is not necessary, but it can improve performance, because it tells Julia to expect 10 values. 
There are other functions such as zeros, ones or fill that can provide a Vector/Array with already filled in data.