Altars are our working surface. It's like an artist's pallet or a carpenter's work bench. It reflects their character and is functional in the aid of their crafts. A witch's altar is just that too. It is a surface from which to work rituals and spells. It can be a special table bought and solely devoted to nothing else or it can be the top of a small box use to hold and carry all the tools. I have had both. When I lived with my parents I had a small end table that I used and stored everything in it's drawer, when I lived in a tiny apartment I had a small wooden box I used because I could move it from room to room. I am currently in the process of setting up a special altar room in my home, but also a spot in my yard! An altar must be big enough to accommodate all of you essential working tools with a little room left over to comfortably allow for special ritual items. A crowded altar can make ritual work a pain and it can destroy your focus. An altar can also be a special cloth you spread out for rituals and then roll up and put away after. Just be sure it is something you feel comfortable working on, especially with lit candles.
Major elements of an altar include the following, your athame ( as well as a wand if you choose to have one), your Goddess candle in the left corner ( I use white or silver), your God candle (I use black or gold), a small cauldron (for incense and ritual use), your altar tile, a water vessel ( I use a small glass bowl), a vessel for salt (again the same kind of bowl as I use for water), a ritual incense holder (one that can be carried), a special plate for cakes and offerings, a chalice or cup for your wine or ritual drink, and an altar cover (usually black but some sabbats or rituals call for a specific color). Many rituals call for specific other tools, so be sure you can accommodate those as well as your regular tools. You want to be able to work around everything without bumping into or knocking over things! You also want to be able to work over it without getting burned on a lit candle or incense. The table doesn't matter but the tools and candles should not be used for anything but rituals. Your Goddess and God candles should only be used for that and you candles used for the quarters should only be used for that. Any candles used for a spell or ritual should not be used for another ritual as you risk mixing energies. Your other working tools ( athame, cup, etc) should only be used for ritual purposes and nothing else, when they are not being used your should store them wrapped in a white or black cloth. All tools should be properly consecrated.
I would suggest keeping things simple and basic for beginners. If you plan to have an altar that remains I would suggest a private placement where guests won't be tempted to paw over your stuff leaving their unwanted energies behind. Other things can be added later on to suit the witch. Your altar is just that, yours!