A working surface, not a shrine you must maintain
An altar is a place where the things you use are kept together and where practice happens. That is most of it. It does not have to be permanent, expensive, symmetrical or photogenic.
What people commonly put on one
Often something for each of the four elements, something representing deity or the season if that suits you, and whatever you are actually working with this week. Many published layouts look elaborate because they were written for coven ritual, not for a shelf.
What you actually need
Nothing. A cleared surface is an altar the moment you use it as one. Everything else can be borrowed from the kitchen.
Building one
Rather than repeating it here, the step-by-step version lives in Practices as Create a simple altar. It is free, it takes about twenty minutes, and it uses things you already own.