The main problem most beginners have is with what to put on the altar. If you belong to a coven or a specific group, the answer to this question may be very specific. If not, the answer could be, "anything!" You will probably collect a lot of "stuff" over the course of your Wiccan career, but not all of it will necessarily have a place on the altar. Keep in mind that the altar is a working space and, as such, should allow you enough room to work. If you put everything you've ever purchased onto the altar, you won't have enough room to work your magick! There are a few basic items that most witches include on their altars:
Altar cloth - Any color, pattern, and cloth type; used to prevent candle wax, saltwater, etc. from damaging the altar's surface and provides a more pleasing appearance.
Candles - See the Candles section.
Elemental representations - For the purpose of the circle, this is usually salt, water, incense, and a lighted charcoal disk, but you can also add things such as soil, seashells, feathers, oil warmers, etc.
God and Goddess representations - Things like antlers, special cups or bowls, figurines, special holders for the god and goddess candles, phallic and vulvar symbols, etc. If you have specific patron deities, you may wish to use a symbol associated with them, or one which your deity suggests to you.
Tools - Generally, an athame, wand, incense burner, cauldron, sword, broom, and fire-proof plate/disk. Items such as the sword and broom, which are too large for the altar, may be placed to the side within arm's reach or under the altar if it's large enough.
"Cakes and ale" - These days, this usually means juice and cookies or crackers, placed in a chalice and on a small plate, respectively.
If you don't have these items, don't panic. Magick is intent, and the Universe isn't concerned with how much "stuff" you have, or how expensive it was. Anything you want to add on top of the basics is perfectly fine, of course, as long as you allow yourself enough room on the altar to work. The altar is your space and you should personalize it. For example, I use a different altar cloth for each season, the material for which I bought at a fabric store. They're beautiful and they were inexpensive (about $2.00 each), and even better, I know that no one else has them. There's nothing wrong with buying generic items at the local occult shop or through popular online stores, but put thought and effort into your purchases. You want the things you use to be items you feel comfortable with. Sometimes it even helps to carry the item(s) around with you until you get used to its energy (after cleansing, of course).
Everything you place on the altar and use for magickal purposes should be cleansed and consecrated. This means you're dedicating it to magick, and that each time you use it, it will be a vessel with which to channel energy. In order to keep it working in peak condition, you shouldn't use it for any other purpose, and you should cleanse it regularly to keep its energy balanced. Some of these things you may feel very attached to and not want anyone else to touch, and that is perfectly fine. If you feel that its energy has been compromised by being handled by someone else, simply cleanse and consecrate it again.
Items should be organized on the altar as follows: earth items in the north, air items in the east, fire items in the south, water items in the west, spirit items in the center, goddess items on the left, god items on the right. For example, I arrange my altar facing north, and so I place my candle lighter (a fire/god item) on my right (in the southeast). I consider my candle snuffer to be a combination of water and earth (goddess energies), both of which extinguish fire, and so I place it on my left (in the northwest). Keep in mind that although placement is important in balancing the energies on your altar, you have to make your tools accessible. You don't want to knock anything over or, worse, burn yourself while reaching for something. See the image below for a visual interpretation.
Some altars remain set up at all times, reminding the witch of the place that magick has in his or her life all the time. However, setting up and taking down the altar with each working can be very useful. Setting up the altar prepares you for the magick you're about to work, and focuses your mid and energy on the task at hand. When taking down the altar, you are returning to ordinary life, and it can ground you in the present (and preventing your "stuff" from getting dusty). Just remember that even though your altar is no longer set up in the physical, it is still intact in other planes, and its energy has not been removed. Even though you can't see it with your eyes, you should be aware of its presence.