A sacred space is merely an area that we choose to devote to our practice. It's kept "clean" of energy we don't want there. It's a focal point, and in some cultures an area to create balance that then reflects in our lives on all levels. When we step into our sacred place, we leave behind our daily lives and step into our magick/spiritual one soley.
If you are not all for ritual work, there are many paths that practice magick while projecting. But having a physical place is like a bridge that gaps the spiritual to the physical for you. Obviously if this is not something you are capable of having, you can consider an outdoor one.
I've found a few sacred spaces of others mapped out with sticks between four trees (one in each direction). A single burnt candle or wax left behind makes it evident. I've also found sacred spaces surrounding a single tree as the center. I walk around them and don't intrude. I find quite a few of them around my area, and it always makes me wonder which neighbors of mine are the magick practioners.
I suggest if you have an outdoor one, you use natural items like feathers, rocks, shells and sticks to "decorate" and create the balance you desire. Gemstones or other small magick items (that aren't expensive) can be buried so they are not stolen by people or other animals that like shiny objects. Remember weather will still damage the material over time, so rather than buy amulets, carve them into wood (wood burning kits make this easy), paint them onto smooth rocks, or draw them on good paper and put the paper along with herbs etc into a baby food jar (they make great little glass jars for odds and ends).
But of course, ones like this are often disturbed by people, animals, and/or weather, so upkeep is more necessary than indoor ones that are protected.
Within ourselves can be and should be a place of peace. And by all means, within is a wonderful place to begin. But a physical sacred space, as I mentioned, is like a bridge. It's a single place we can step into and immediately feel that magick/spiritual environment that is familiar and puts us in the right frame of mind.
When I first started out, I didn't have a sacred space. I had a large box with all my things in them and that was my "space". Upon sitting down and opening the box, I felt washed over by the feeling of that beautiful energy and was ready to use them for whatever it was I was planning.