In art, the support is just jargon for the thing you paint on. Canvas is a traditional support, but so is wood panel. Canvas needs to be stretched over stretchers (or purchased pre-stretched) or glued over a piece of flat wood or particle board to provide stiffness. After that, paint on some acrylic gesso to make a good surface for your painting and collage.
Once the gesso dries, you can start right in with the acrylic painting. Use colors as you wish. Acrylics dry quickly, and at similar speeds, so there are no technical limitations to mixing colors. You may not want to build up heavy texture at first, as it can be harder to adhere collage materials to an irregular surface, but as long as you don't mind that some edges of the paper may not be completely adhered, go right ahead with thick textures.
Paint the matte medium onto your collage material, and stick it right onto the acrylic paint. You should use porous materials like paper or cardboard. You can employ photos, magazine pages, printed paper or computer printouts, for example. The acrylic paint you adhere to the collage material to can be wet or dry; the matte medium will still make a solid bond. Give the matte medium about 30 minutes to dry for the best result.
You can continue to paint on or over the collage material, or add more collage material to the painted areas, in any fashion you wish. The paints and medium are totally compatible, so you can build up dense and complex surfaces or go minimal, depending on your taste.