How do I add a column to an existing table.

I have this table I call "new_users". The existing first col of that
table is "first_name". I want to add an additional column in FRONT of
the existing first column of "first_name"...

Existing colums... first_name | last_name | <etc>

I want to add... key | first_name | last_name | <etc>

How is that done, then I want to automatically insert into "key" a
sequence of numbers like 1,2,3,4,5,6,7, etc so that each number is
sequentially numbered.

I'm very new to SQL so I suppose this is prolly a pretty dumb question.
The docs on ALTER show how to add a column, but NOT how to add the column
to the beginning, or after adding a column, to re-arrange them so that
the KEY column is first. Could someone please help me?

I have another related question. I know it is possible to "Join" tables,
creating a 3rd table. But can I take this 3rd table and add this table
to my list of tables in my database? If so, now do I do that?

John