casting a circle is a basic art
here is a link to an article circle casting: http://www.spellsofmagic.com/coven_ritual.html?ritual=1298&coven=6
an alter is a sacred space that has items on it to represent the elements, your patron gods and any other tools you use in your craft. the position of these items doesnt matter as this is your own sacred space so feel free to put them in any position you think is best.
hope this helped