You'll want to choose what evergreens you want to use to green in your Christmas tree centerpiece. I'm lucky to be able to cut and harvest from my own backyard, since I have an abundance of natural evergreens and bushes to use from.
If you cut your own, gather in the cool of the day and give the cut greens a soak in a tepid bath for about 20 minutes. Allow to dry, wrap lightly in plastic and store in a cool area (even outdoors) until you are ready to use them. Do not allow them to freeze - but evergreens are pretty hardy.
I'm using yew trimmed from a large hedge in my yard. I love the color, but it really doesn't have a heavy "christmas" scent like fresh cut pine. But I think pine would have too much of a "bushy" look for my taste, so I'm going to green in with the yew.
If you don't have your own shrubs or trees to harvest from, don't fear. You can buy a mix of greenery in bulk just as you do with wholesale flowers.
I recommend that you choose one entire variety to green your tree, unless you really like the mixed look. I think fir, hemlock, or cedar would be a good design choice for this particular centerpiece. I've often seen them made completely with boxwood as well.