Each homeschooling family has a unique situation in regards to their "classroom." Some families are fortunate enough to have an spare room that can be used for schooling purposes, while others will have to use a corner of the living room or the kitchen table! Any of these situations can work well.
The most important thing is to make the area comfortable for the parent and children and to have the children involved in the project!
WORK SURFACE
The first consideration is what type of work surface is available. I recommend going to your local office supplier and purchasing a large,sturdy folding table. Not only are these economical (usually around $40-50), they provide plenty of room for your child(ren) to store supplies in their own area and have more work space than is provided by a standard student desk.
Of course, a kitchen table is sufficient if you do not mind having to empty everything off of it once it is needed for a meal!
Be sure that the chair the student has is well suited for their size and the table.
LIGHTING
Lighting is critical. Open windows and a bright ceiling fixture are helpful. If that is not enough, a desk lamp is an inexpensive help.
SHELVES/ORGANIZATION
Bookshelves are critical. As your homeschooling journey continues, you will collect curriculum and books that need a well organized home. Milk crates are helpful for containing each student's curriculum and belongings (a different color for each child will avoid confusion). Also, a large accordion file for mom to keep up with quality work that needs to be saved can be very handy. Notebooks for each subject are also invaluable.
BOOKS
Every classroom needs basics such as a dictionary and thesaurus, as well as classics that are age appropriate. Check your local friends of the library, used book stores, and watch ebay for bargains!
EXTRAS
A trip to your local teacher supply company can yield helpful learning posters (for example, the steps of the scientific method or parts of speech) and maps. It is great to have both a US and a world map to hang on a wall so that as you discuss any subject, you can incorporate geography. Maps can be tied into reading, history, Bible Study, etc...
Another fun thing to do is create a timeline around the top of your classroom to which you glue on a reminder in the appropriate spot as you study a person, historical event, art work, or read a book set in a particular time period. (Find a picture on the internet that can be shrunk and printed for your children to color to add to the timeline!)
These things help the kids make the classroom their own, remind them of how much they've learned, and make learning fun.
Finally, do not be afraid to step outside of the box. Go to the backyard and have class on a blanket on a pretty day. Pack a picnic and go to the park. Go for a nature walk for science class.