Opening a circle
It is preferrable to mark a permanent circle in a permanent place either on the floor, on a piece of carpet that you can roll up and put away or best yet - outdoors in a quiet secluded place. Most, if not all, of these options are rarely available; especially if you live in a city or have house-mates. So remember that although those options are the ideal, visualizing your circle around you is just as effective.
To cast the circle, first make sure that you have all of the tools you will need for your spell inside the circle you'll be creating. Because once you cast the circle, do not break the circle. As a strict rule, never break the circle during a spell. Not only would your magickal energies "leak out" of the circle (and misdirected magickal energy can be very dangerous), but you will be very vulnerable to outside negative energies in your "mid-spell"-state. One word : DON'T!!!
Your altar should be set up with you facing the altar in the direction associated with your ritual (money spells facing the north, love spells to the west...etc....)
Now light a white candle (white is a color of protection) and meditate/concentrate on what you want to accomplish with your spell. When you've "gotten into the right frame of mind," Picture your circle forming from the point directly in front of you (on the other side of your altar) deosil (clockwise) all the way around you, completing back in the west. You can visualize this as a white light forming a ring around you or even as a flame - as if it were a ring of gasoline lit with a match at the point of the circle directly in front of you. Once the circle is completed, visualize the light extending slowly upward from the circle to form a translucent "dome" or "bubble" of light around and above you - completely enclosing you inside. Now work your spell.