A ' circle' must ALWAYS be cast to do any and ALL magic, spells, rites, rituals etc etc. The circle is the 'sacred' space where the magic begins and ends. Use a wand your finger or even your own mind if it's strong enough to maintain the circle around you. It keeps things out from your work which could cause problems with any magical thing you do......
I won't disagree with the uses of a circle as SpellcasterI stated. But I personally see a circle as an optional choice; like a tool. And as such it's a matter of deciding whether or not to use one. Personally I don't use circles, but I have used a circle once and did not much care for it.
Aside from the start and end of magickal workings, a circle is also used as a method of protection.
I suggest going with your gut feeling and determining from that whether or to use one. It may also help to try it with your practices and find out if you are comfortable with the casting of a circle.
A simple, easy method for this is to extend out your arm and pointer finger. Pointing at the ground visualize a beam of light shooting out and touching on the ground where you point. Turn in a complete circle; the beam constantly shooting out of your finger, and when you make a full circle you have cast your circle. You could also use a wand. I've heard of some people using an athame for this process (though I am sure some one may find that as an improper usage of the tool)
You could just visualize your circle around you. The only downfall with this is that you would need to keep constant at visualizing the circle to keep it up and working. Unless your good enough with this to only consciously visualize it being there and then your subconscious mind taking care of keeping it up and working.
Another method is by starting at facing the North, South, Easy, or West (starting point is debatable but what matters is making the full circle) and then calling upon the element of that quarter. Keeping going round and calling upon the elements until you make the full circle and you're back at the starting point. At each quarter you could ask the elements to not only add their part fort the circle but also stuff like protection, aid in your magickal workings, extra energy, etc. It's how you want it to be. (Though some people would debate that you call upon them for a specific reason in a specific way)
You could also do a more physical, tangible, human eye visible circle. Make a circle with stuff like chalk, salt, rocks, candles, or sticks.
It is debatable as to which way you turn making the circle. Some say clockwise to make and counter to break. I personally don't see a difference in whether you turn left or right. It's what you're comfortable with doing. What works best for you. Just as long as you make the full circle. And when you make the circle you want to make it so you have plenty of room to get done what all you need or intend to do.
The use of circles is personal preferrence. I would highly recommend using one only if you are working with or against spiritual beings. I also like casting a circle when divinating on important subjects to keep out unwanted spiritual influence.
A circle can also be used if you have trouble raising power.
However, consecrated/sacred space is important. You always want to keep the energy around where you work "clean" to prevent unwanted foreign energy lurking about.
I think of a circle more as, like mentioned before, a space of cleansed energy to do your work. I don't believe it's essentially necessary to do it for every ritual but I highly suggest doing some sort of cleansing or such in the space you're working to make sure nothing interferes with your energy and what you're trying to accomplish.
It depends on how you do it. When I cast a circle, I use smoke (smudging) to trace my circle and then come to the center of it and bless/honor the 6 directions (east, north, south, west, above and below). This holds a dual purpose of creating a boundary and creating sacred space. You can also sprinkle blessed water if fire is an issue.