The artist term for what you paint on is called a support or ground.
First – make sure the canvas is double-primed for acrylic. This means that the canvas is treated with two coats of primer (called gesso) so that the paint will not soak into the fibers of the canvas. Canvases primed for oil will not work with acrylic since the oil primer will repel the acrylic paint.
Or…you can prime the canvas yourself. (I’m much to lazy to do this.) Apply a coat of gesso horizontally and let it dry for 24 hours, then lightly sand the surface smooth. Then apply a second coat vertically, let dry for another 24 hours and then lightly sand.
I also recommend that you only buy cardboard-backed canvas for practicing. Never use it for a finished piece. The cardboard will absorb moisture and can warp. Worse yet, the moisture can become trapped between the cardboard and the waterproof dry acrylic paint and the entire piece may mold. YUCK.
So, you should purchase canvas that is either gallery wrapped or museum wrapped. These canvases are stretched over a wooden frame and can breathe from the front and back. If you paint the sides (and I always do) they can be hung without framing. Museum wrapped canvases have thicker sides than gallery wrap.
Canvases are like sheets, the tighter the weave and the higher quality the material, the higher the price.
I like to paint on 16 x 20″ canvas. It is large enough to allow me to include some detail but small enough for me to complete a painting relatively quickly. You can experiment with different sizes.
Speaking of experimenting, you can actually paint on anything the acrylic will stick to, although some surfaces may require priming with gesso. Be creative with your supports and by all means, have fun.