The altar is a place of worship, condensed power, and operative function. It can take any number of shapes, but the shape it takes depends heavily upon the nature of the works you're looking to do. I'd also suggest considering the shape the work you're planning to do, as that should direct your decision-making process.
A Meditational Altar:
For instance, if you're looking to establish a place wherein you're able to meditate and contemplate at the end of a long day, you really don't need much. I'd recommend finding a small table that allows you to easily see the altarspace and what's upon it while sitting...a coffee table or small end table would do. Upon it, I would recommend placing a visual focus and two pillar candles to be lit when you begin your meditations and extinguished upon your completion. Between those candles, place an incense burner, and behind it an image of whatever it is you'd like to meditate upon.
This can then be consecrated to serve as a meditational focus that assists in advancing your progress. It might not seem like much, but it makes a world of difference and can act as a kind of safe abode for internal exploration.
A Christian Altar:
If, however, you're a Christian and wish for a sanctified place upon which you can set vigil candles, make offerings to YHVH & invoke angels, you'll construct it somewhat differently. I would recommend procuring a statue of a Crucifix, holy water, church incense, an incense burner, a white cloth, white candles, candle holders, and your bible.
Set the Crucifix before you, towards the rear of the altar space, and set the white cloth before the Cross. Upon it place the Bible, placing the incense burner between the Crucifix and the Bible. The candles and holders can be placed around the Crucifix to form a square. Then, sanctify the space with holy water by blessing the four corners of the altar whilst orating the Lord's Prayer in Latin.
Then, whenever you feel like you're under duress and could use some divine succor, return to this space, crack the Bible open to the psalm or passage you're interested in, light incense and read the passage aloud. Recognize that Sandalphon will hear your prayers and send them up through the smoke, and use this as a daily or weekly place of worship that can be of great meaning to you.
A Daemonolator's Altar:
To those of you that are more inclined towards working with powers on the opposite end of the spectrum, I would suggest procuring a black cloth, four pieces of iron, and an image of the infernal. Place the cloth in the center of the altar and set the four pieces of iron at its respective corners. Set the seal of the power you wish to work with in the center of the cloth, with an image of the infernal behind it.
When you then feel the need to engage in daemonolatry, come to this space and use the being's enn or invoke them through your own oration. Light for them one red candle and one black candle and connect with them as you see fit.
If you feel inclined to increase the power and potency of the manifestation in play, find a metal bowl, fill it with sand, and place upon it lit charcoal and the appropriate raw incense. Use this as a kind of conjurative focus that brings greater manifestations into the space.
A Witchy Workspace:
If, however, you're the sort to lean less towards the heavens or hells, and more towards the Earth and its powers, you can bring that into your space as well. Procure for yourself an earthenware bowl, dry earth from your area. beeswax, tobacco, and copal, and a small receptacle that fits inside the earthenware bowl. Each day, mix up offerings for the powers of place, ensuring that the offering will fit inside the receptacle inside the bowl, being mindful of what you're looking to achieve that day.
You're welcome to place four candles about the bowl, but I would instead recommend being mindful of the elemental nature of the work you seek to do. When invoking powers of Water, purify water and offer it to those powers. When invoking powers of Fire, use a fire-safe receptacle and light a candle to them. Air?...incense. Earth?...fresh flowers.
An HGA Altar:
Finally, I'll talk a bit about an HGA altar, as that's the form of this I find to be most interesting and rewarding, personally.
For those of you who have K&C with your Holy Guardian Angel, this will be relatively simple. For those of you who have not yet undergone either the Abramelin Operation or some variation thereof, you can adapt this to serve you in that purpose.
Create a sigil or image that to you encompasses the essence of your HGA and make that the centerpiece of your altarspace. Under it place a deep purple cloth shroud, then collect and place upon your altar items that speak to you (and your HGA). They can be crystals, sharp things, candles, witchy tools and herbs, the works...its all you, but you want to make sure that this space is sanctified according to your needs and wants.
I generally like using the Stele of Jeu as a manner of invocation when creating and consecrating this kind of space, as its tech that can be reoriented towards one's higher operative powers with relative ease. If you get it right, this place should sing to every aspect of your being...you'll feel it.
The whole point is to get creative while having a place that works for you and your spirits. You can get as inventive as you like and use that interest to show care and consideration for the spirits. It earns some serious brownie points and generally streamlines the magicospiritual process.
Just a couple of inspirational ideas to get you started. ;)