Outside of any particular religion, an altar in itself is a space for connection and gratitude, and serves as a place of practice. There are many different reasons why we might build one - to celebrate the change in seasons, to worship the moon or a specific planet or deity, to perform ritual magic, or simply as a space of meditation and reflection. My altars have taken many forms in the years since I started building them, and I have come to think of them as a physical representation of the metaphysical & spiritual worlds to which I am connected.
When I first built an altar it was probably based very much on a wiccan or pagan altar used for magic - it this type of altar the items will be placed in a certain order, typically corresponding with the directions and elements that go with them. I usually still place a chalice to the West for Water, my incense to the East for Air, but it has grown to encompass so much more than that, and I now have a collection of altar items that I can play around with. I love to move them around and set them up for different holidays or rituals.
I just moved into a new house. My previous altar was in need of a good energetic cleaning, so I took the time to carefully remove and pack everything away, cleaning and thanking the items as I went. And when it was time to rebuild, there were certain placements I kept and lots of items I put away. It's generally a good idea to refresh your space every so often, whether you do it regularly as your practice changes, or whether you keep the same altar over a long period of time, cleaning the space is important. Removing dust and clutter, wiping things down, replacing dead flowers with fresh ones, and performing some sort of energetic cleanse using incense, sage, or another means of clearing any old or stagnant energies. And as we take time from our often busy, real world lives to do this, we offer ourselves in devotion to the mysteries and path we follow.
And they are so fun and easy to create! When I was growing up I collected lots of nick knacks, and my mom was always gifting me with small treasures I kept around my room. Over the years I fell in love with crystals and candles, and collected other magical supplies. But your alter can be whatever you want it to be. At one point late last year, my altar was covered in small bones I found buried in my backyard, pictures of long dead relatives, and dirt, twigs & sticks from outside (that altar was built for an online Avalon Samhain Mysterium I signed up for last year - it was definitely very different than my usually sparkly, colorful, feminine altars).
So work with what you've got, as you feel called at the time. In fact, a super wonderful way to connect in with your intuition and root into a spiritual practice, is the act of building a sacred space. Start with where you're going to build it. You can use a windowsill, a small table. I've currently got one altar that is an overturned storage bin, and another that is a vintage footstool. I've draped them both with scarves I never wear, and positioned one sticking out from the other. I love creating depth and having multiple connected levels.
Look around your home and find things that are special to you. Or things that you feel drawn to. Or think about what you want to call into your life, and create an altar that acts in resonance with that. Looking to bring more love into your life? Think hearts, roses, pink & red crystals. And don't second guess your choices either. Learning to trust our intuition means learning to trust that very first insight, no matter how fleeting it may have been.