So before you go hunting for all those items you want to buy, relax! First decide the best location for your altar. It should be one that is relatively "safe" - meaning a place that isn't in the main traffic zone of your home where it might be bumped or disturbed (or become a place to dump mail or keys) or where your children or pets will think its the perfect spot to play. And remember, you can have more than one altar depending on your path and what you want to use them for - an ancestral altar, meditation altar, or moon phase altar are all great supplements to your main, working, ritual altar. Perhaps you will want to create your ritual altar fresh every time you need it (practical if you don't want everyone in your house to know what you are practicing) instead of being set up permanently. So decide what the main focus of your altar will be (and don't be afraid to change it as your practice evolves).
Once you find a place and decide the purpose of it, start with a simple altar cloth - measure the flat surface you are planning to use and take a trip to the fabric store. You can often find remnants of very beautiful fabrics for just a dollar or two. Get some fabric paints if you'd like and decorate a plain cloth. Get several fabrics of different colors if you want to set up a seasonal altar. Let your intuition guide you and have some fun with it - your altar doesn't have to be serious!
Now that you have a cloth, most altars have 1 thing for each of the elements of the Craft - Earth, Fire, Air, Water and Spirit. Look around for things you already have that mean something special to you - they will already have great energy. Perhaps your grandmother's candy bowl is perfect for holding water or that beautiful sea shell you found on your trip last summer could represent water. Small fountains are also perfect adding the beautiful sound of water to your rituals.
Candles are wonderful, but not always practical depending on where you set up your altar or if you have cats that can knock them over (one of my cats set herself on fire and never knew it by brushing against my candle at the wrong time - I freaked more than she did and she only singed a few hairs). Perhaps a night light or electric Christmas candle would be a better fit for fire for you.
Air is usually represented by incense, but a feather found on a spring walk would do as well. A pretty potted plant or found rock or pine cone works well for Earth. Spirit could be represented by a candle, a pentacle (how about one made from twigs from the nearby park!), or any image of a God or Goddess that you like.
Beyond the basics, you might want to add special objects depending on the purpose of your altar. Ancestral altars are easy - photos or mementos of members of your family in pretty frames with a candle or offering dish is all you need.
Moon altars could have any shiny, round object to show the moon - for years I used a silver blown glass Christmas ornament with the hook and cap removed sitting in a tapered vase - I covered it with a black cloth completely during the dark of the moon, partially for other phases and let it shine uncovered during full moons.
Meditation altars can have anything that helps you focus - a statue, incense, candle or mirror are some ideas.
If setting an altar for a specific ritual or spell, again take a look around and figure out what represents the purpose of the ritual best to you - it will automatically be more powerful and just the process of considering objects and designing your altar can be magical.
If you are not going to let the altar set up permanently, find a pretty box to store everything in when not in use. Craft stores have inexpensive plain wood boxes of all sizes you can get and paint or decorate as you'd like. Perhaps you'll want to get several and store seasonal altars in their own boxes for when you need them.
Books are wonderful, especially when just starting out, but don't get too bogged down in what they say you "should" do - treat them as guidelines and let your own intuition be your true guide. Don't be afraid to mix things up and change them from time to time either. Enjoy the process and your magic will be that much more special.