Magic Forums

Hey everyone i am new to all of this and there are a couple of things i would like to know about. Having read through countless spells and through many questions on the forums, i have come across 2 things that i know nothing about.

In order to cast spells, it is recommended that you cast a circle. Can someone explain the purpose of this circle and how i create it? Also there is mention of using an altar. I do of course know what an altar is but is there something specific you have to use like a table?

Any advice on both of these would be greatly appreciated. The more detailed the better. Thanx for your time