There are certain considerations that you must take into account when deciding where to place your altar. If possible it should be in a space where it can remain undisturbed. If you leave your altar set up all the time, and you do not live alone, you must let your housemates know that the items on your altar are not objects of curiosity or stage props, and should not be handled. Ideally, no one should touch your ritual tools except you. If your altar is going to be just another coffee table filled with wierd looking stuff to paw over, you should consider either placing it in your private portion of the premises, or not leaving it set up between workings.
If you have more understanding housemates, or none at all, setting up the altar in either a private or public position in the home depends upon your lifestyle. Are you 'out' as a witch to your friends and family? If so, it's presence won't bring about difficult questions when your parents, who "never come over", decide to stop by. Some of the acoutrements of an altar are easy to explain, a vase of flowers and some candles are home decor, but the athame and that great copper wand with the huge quartz crystal you found at that funky shop aren't. Of course alot depends on how theatrical a witch you are. If you love the atmosphere created by massive candlesticks, linen altar cloths, loads of crystals, images of the Goddess and all the trappings, it's going to be harder to hide than if your more minimalist.
Now that you have decided on public, livingroom, vs. private, bedroom, and whether to leave it up or put things away each time, it's time to decide what sort of altar you want. This is in part based on what sort of witch you are, theatrical, minimalist, or somewhere in between. How much stage dressing do you like? If the decorations are useful in helping you get into the right frame of mind for magic then by all means, go for the props. You're carving out a spot for your magic from the mundane world around you, there are distractions in that world and if it helps your concentration to have a temple that looks like a Hollywood set, there's nothing wrong with that. If on the other hand, your body is your altar and your mind control is complete you really don't need much more than a flat surface to put some candles on once in a while.
Earlier I mentioned a coffee table as a potential altar, and in fact they make great ones. Most are large enough to put your ritual tools and a few candles on easily, and they are fairly cheap at garage sales or the Goodwill. With a coffee table altar you work seated before it after casting the circle. If you prefer to work standing, look for a taller table such as an old dinette table or large end table. It's rather nice if the table has a drawer to put your things in, but it's not required. If you, or a friend, are handy with tools you can easily build a simple altar for yourself, to your own design. Whatever table, or crate, you end up with, it's nice to consecrate it to it's new purpose, and purify it. Smudging with sage, ritual purification, exposing it to the sun or moon, all will work.
Now you have your altar, and some idea of where you want to put it, you must orient it to the correct cardinal point for your tradition. In mine this is North, altars always face North, but in others it is East. Use a compass if you are not already familiar with the directions where you live. Also, depending upon the tradition, the altar may be at the edge of the circle or in the center. In mine it is at the edge. Take this into consideration since if it is to be in the center, you will need enough space to pass on all sides of the altar. If it is to be at the edge, you can place it against a wall facing in the correct direction. When working as a solitaire you do not need to cast as large a circle as you must when working with a coven. This must be considered as well, is this your personal altar or will your coven be gathering at your home and using it for group work? This will determine how much room you need surrounding the altar.
Once you have the altar physically
set up and oriented it is time to consider what will be on it. This may
vary somewhat depending upon the sort of magical working being performed,
but in general, the usual items include the ritual tools you work with,
altar candles, a censer, and perhaps your Book of Shadows or a spell book
of some sort. The simple diagram below illustrates this basic layout.
The colors indicate the elemental correspondences of the items, red=fire, yellow=air, blue=water, green=earth.
It is important that you put together an altar you like the look and feel of. It should please you, not your best friend, not your High Priestess, not your coven members, you. You have to work with it, not them. This is a place for individual expression. If you love Maxfield Parrish, and want to use "Ecstasy" as a Goddess image, do it. If you want to make altar cloths in each of the planetary colors, do so. Also, don't forget music. Some people insist on complete silence, but others love Celtic music or Yanni (yuck!) in the background. Suit yourself. A CD player can be a sort of ritual tool if you want it to!