The purpose for casting a circle is to create a magickal space that is sacred and protected for your spells and rituals. Not all traditions cast a circle, and I would consider it an option for spellcasting rather than a requirement. Though the specifics can vary from coven to coven, this is the general approach for casting a circle.
1. Mark out your circle space. A little chalk or a few candles can physically create a circle, or you can just mentally visualize it. If you are working in a group, it needs to be large enough that everyone can fit inside.
2. Call the quarters. They are sometimes referred to as the Watchtowers, but are generally just the elemental spirits of the four directions. Call on each one and ask them to watch over your circle. I have another page on calling the quarters that can explain the traditional wording and approach.
3. Once you create your circle, you will perform your spell inside. But in a group, the person who cast the circle will one-by-one allow the rest of the group to enter before you can get started. The leader asks each person's intention on entering, and the traditional reply is "in perfect love and perfect trust". In some cases, you can "cut" a doorway in the circle with an athame to allow entrance without disturbing the spiritual energy.
4. At this point, you can cast your spells or perform your rituals inside your circle. The purpose of the circle is to both keep your energy inside, as well as to keep any negative energy outside. It's not really about evil spirits or demons, or anything else like that you might see in movies.
5. After your spell is over, you have to release the circle. The first step is to bid farewell to the Watchtowers you called earlier. Just go through all 4 directions and thank them for their help, and allow them to leave. Then visualize your circle opening up and the power dispersing.
Whether you choose to cast a circle with your spells is up to you. It can add further power to your work but it is by no means a necessity. Maybe you can add this to your routine once you have a few other simple spells under your belt and a little magickal experience.