VB is so old school, who teaches that anymore? even in HS these days, students are taught programming with java or some open source language.
Declaring a variable reserves a name and some space in memory for a variable of the specified type, but doesn't give it a value. Initializing gives the variable a value. Depending on how you're going to use the variable, you can declare it without initializing it and then initialize it later, or declare and initialize at the same time.
// declares integer variable
Dim num1 As Integer
// initializes integer variable that's already been declared
num1 = 6;
// declares and initializes integer variable at the same time
Dim num1 As Integer = 5;
Stack over flow is a great resource for programing questions.
An array is a list of items, a bit like a shopping list. It allows you to store more than one item in only one variable.Think of it like this. When writing your shopping list, you could use a separate piece of paper for each item you need to buy (a variable). However this is silly and unneeded—could you imagine how hard it would be to carry all that paper around with you? So, you use one piece of paper for all of your items. This one piece of paper is your array.
Youtube is a good resource also for tutorials on this stuff.
here's a video on arrays: https://www.youtube.com/watch?v=gKiNGZMuqI0